X-Git-Url: https://wannabe.guru.org/gitweb/?a=blobdiff_plain;f=datetime_utils.py;h=b05097a909a53a1a39836dffbd91e02887b8aabd;hb=532df2c5b57c7517dfb3dddd8c1358fbadf8baf3;hp=d70bf4a79008effe5e2a3aec8684d3352be6c78d;hpb=497fb9e21f45ec08e1486abaee6dfa7b20b8a691;p=python_utils.git

diff --git a/datetime_utils.py b/datetime_utils.py
index d70bf4a..b05097a 100644
--- a/datetime_utils.py
+++ b/datetime_utils.py
@@ -1,12 +1,16 @@
 #!/usr/bin/env python3
 
+# © Copyright 2021-2022, Scott Gasch
+
 """Utilities related to dates and times and datetimes."""
 
 import datetime
+import enum
 import logging
 import re
-from typing import NewType
+from typing import Any, NewType, Optional, Tuple
 
+import holidays  # type: ignore
 import pytz
 
 import constants
@@ -14,29 +18,541 @@ import constants
 logger = logging.getLogger(__name__)
 
 
-def now_pst() -> datetime.datetime:
-    return datetime.datetime.now(tz=pytz.timezone("US/Pacific"))
+def is_timezone_aware(dt: datetime.datetime) -> bool:
+    """See: https://docs.python.org/3/library/datetime.html
+                               #determining-if-an-object-is-aware-or-naive
+
+    >>> is_timezone_aware(datetime.datetime.now())
+    False
+
+    >>> is_timezone_aware(now_pacific())
+    True
+
+    """
+    return dt.tzinfo is not None and dt.tzinfo.utcoffset(dt) is not None
+
+
+def is_timezone_naive(dt: datetime.datetime) -> bool:
+    """Inverse of is_timezone_aware.
+
+    >>> is_timezone_naive(datetime.datetime.now())
+    True
+
+    >>> is_timezone_naive(now_pacific())
+    False
+
+    """
+    return not is_timezone_aware(dt)
+
+
+def strip_timezone(dt: datetime.datetime) -> datetime.datetime:
+    """Remove the timezone from a datetime.  Does not change the
+    hours, minutes, seconds, months, days, years, etc... Thus the
+    instant to which this timestamp refers will change.  Silently
+    ignores datetimes which are already timezone naive.
+
+    >>> now = now_pacific()
+    >>> now.tzinfo == None
+    False
+
+    >>> dt = strip_timezone(now)
+    >>> dt == now
+    False
+
+    >>> dt.tzinfo == None
+    True
+
+    >>> dt.hour == now.hour
+    True
+
+    """
+    if is_timezone_naive(dt):
+        return dt
+    return replace_timezone(dt, None)
+
+
+def add_timezone(dt: datetime.datetime, tz: datetime.tzinfo) -> datetime.datetime:
+    """
+    Adds a timezone to a timezone naive datetime.  This does not
+    change the instant to which the timestamp refers.  See also:
+    replace_timezone.
+
+    >>> now = datetime.datetime.now()
+    >>> is_timezone_aware(now)
+    False
+
+    >>> now_pacific = add_timezone(now, pytz.timezone('US/Pacific'))
+    >>> is_timezone_aware(now_pacific)
+    True
+
+    >>> now.hour == now_pacific.hour
+    True
+    >>> now.minute == now_pacific.minute
+    True
+
+    """
+
+    # This doesn't work, tz requires a timezone naive dt.  Two options
+    # here:
+    #     1. Use strip_timezone and try again.
+    #     2. Replace the timezone on your dt object via replace_timezone.
+    #        Be aware that this changes the instant to which the dt refers
+    #        and, further, can introduce weirdness like UTC offsets that
+    #        are weird (e.g. not an even multiple of an hour, etc...)
+    if is_timezone_aware(dt):
+        if dt.tzinfo == tz:
+            return dt
+        raise Exception(
+            f'{dt} is already timezone aware; use replace_timezone or translate_timezone '
+            + 'depending on the semantics you want.'
+        )
+    return dt.replace(tzinfo=tz)
+
+
+def replace_timezone(dt: datetime.datetime, tz: Optional[datetime.tzinfo]) -> datetime.datetime:
+    """Replaces the timezone on a timezone aware datetime object directly
+    (leaving the year, month, day, hour, minute, second, micro,
+    etc... alone).
+
+    Works with timezone aware and timezone naive dts but for the
+    latter it is probably better to use add_timezone or just create it
+    with a tz parameter.  Using this can have weird side effects like
+    UTC offsets that are not an even multiple of an hour, etc...
+
+    Note: this changes the instant to which this dt refers.
+
+    >>> from pytz import UTC
+    >>> d = now_pacific()
+    >>> d.tzinfo.tzname(d)[0]     # Note: could be PST or PDT
+    'P'
+    >>> h = d.hour
+    >>> o = replace_timezone(d, UTC)
+    >>> o.tzinfo.tzname(o)
+    'UTC'
+    >>> o.hour == h
+    True
+
+    """
+    if is_timezone_aware(dt):
+        logger.warning(
+            '%s already has a timezone; klobbering it anyway.\n  Be aware that this operation changed the instant to which the object refers.',
+            dt,
+        )
+        return datetime.datetime(
+            dt.year,
+            dt.month,
+            dt.day,
+            dt.hour,
+            dt.minute,
+            dt.second,
+            dt.microsecond,
+            tzinfo=tz,
+        )
+    else:
+        if tz:
+            return add_timezone(dt, tz)
+        else:
+            return dt
+
+
+def replace_time_timezone(t: datetime.time, tz: datetime.tzinfo) -> datetime.time:
+    """
+    Replaces the timezone on a datetime.time directly without performing
+    any translation.  Note that, as above, this will change the instant
+    to which the time refers.
+
+    >>> t = datetime.time(8, 15, 12, 0, pytz.UTC)
+    >>> t.tzname()
+    'UTC'
+
+    >>> t = replace_time_timezone(t, pytz.timezone('US/Pacific'))
+    >>> t.tzname()
+    'US/Pacific'
+
+    """
+    return t.replace(tzinfo=tz)
+
+
+def translate_timezone(dt: datetime.datetime, tz: datetime.tzinfo) -> datetime.datetime:
+    """
+    Translates dt into a different timezone by adjusting the year, month,
+    day, hour, minute, second, micro, etc... appropriately.  The returned
+    dt is the same instant in another timezone.
+
+    >>> from pytz import UTC
+    >>> d = now_pacific()
+    >>> d.tzinfo.tzname(d)[0]     # Note: could be PST or PDT
+    'P'
+    >>> h = d.hour
+    >>> o = translate_timezone(d, UTC)
+    >>> o.tzinfo.tzname(o)
+    'UTC'
+    >>> o.hour == h
+    False
+
+    """
+    return dt.replace(tzinfo=None).astimezone(tz=tz)
 
 
 def now() -> datetime.datetime:
+    """
+    What time is it?  Returned as a timezone naive datetime.
+    """
     return datetime.datetime.now()
 
 
-def datetime_to_string(
+def now_pacific() -> datetime.datetime:
+    """
+    What time is it?  Result in US/Pacific time (PST/PDT)
+    """
+    return datetime.datetime.now(pytz.timezone("US/Pacific"))
+
+
+def date_to_datetime(date: datetime.date) -> datetime.datetime:
+    """
+    Given a date, return a datetime with hour/min/sec zero (midnight)
+
+    >>> import datetime
+    >>> date_to_datetime(datetime.date(2021, 12, 25))
+    datetime.datetime(2021, 12, 25, 0, 0)
+
+    """
+    return datetime.datetime(date.year, date.month, date.day, 0, 0, 0, 0)
+
+
+def time_to_datetime_today(time: datetime.time) -> datetime.datetime:
+    """
+    Given a time, returns that time as a datetime with a date component
+    set based on the current date.  If the time passed is timezone aware,
+    the resulting datetime will also be (and will use the same tzinfo).
+    If the time is timezone naive, the datetime returned will be too.
+
+    >>> t = datetime.time(13, 14, 0)
+    >>> d = now_pacific().date()
+    >>> dt = time_to_datetime_today(t)
+    >>> dt.date() == d
+    True
+
+    >>> dt.time() == t
+    True
+
+    >>> dt.tzinfo == t.tzinfo
+    True
+
+    >>> dt.tzinfo == None
+    True
+
+    >>> t = datetime.time(8, 15, 12, 0, pytz.UTC)
+    >>> t.tzinfo == None
+    False
+
+    >>> dt = time_to_datetime_today(t)
+    >>> dt.tzinfo == None
+    False
+
+    """
+    tz = time.tzinfo
+    return datetime.datetime.combine(now_pacific(), time, tz)
+
+
+def date_and_time_to_datetime(date: datetime.date, time: datetime.time) -> datetime.datetime:
+    """
+    Given a date and time, merge them and return a datetime.
+
+    >>> import datetime
+    >>> d = datetime.date(2021, 12, 25)
+    >>> t = datetime.time(12, 30, 0, 0)
+    >>> date_and_time_to_datetime(d, t)
+    datetime.datetime(2021, 12, 25, 12, 30)
+
+    """
+    return datetime.datetime(
+        date.year,
+        date.month,
+        date.day,
+        time.hour,
+        time.minute,
+        time.second,
+        time.microsecond,
+    )
+
+
+def datetime_to_date_and_time(
     dt: datetime.datetime,
+) -> Tuple[datetime.date, datetime.time]:
+    """Return the component date and time objects of a datetime.
+
+    >>> import datetime
+    >>> dt = datetime.datetime(2021, 12, 25, 12, 30)
+    >>> (d, t) = datetime_to_date_and_time(dt)
+    >>> d
+    datetime.date(2021, 12, 25)
+    >>> t
+    datetime.time(12, 30)
+
+    """
+    return (dt.date(), dt.timetz())
+
+
+def datetime_to_date(dt: datetime.datetime) -> datetime.date:
+    """Return the date part of a datetime.
+
+    >>> import datetime
+    >>> dt = datetime.datetime(2021, 12, 25, 12, 30)
+    >>> datetime_to_date(dt)
+    datetime.date(2021, 12, 25)
+
+    """
+    return datetime_to_date_and_time(dt)[0]
+
+
+def datetime_to_time(dt: datetime.datetime) -> datetime.time:
+    """Return the time part of a datetime.
+
+    >>> import datetime
+    >>> dt = datetime.datetime(2021, 12, 25, 12, 30)
+    >>> datetime_to_time(dt)
+    datetime.time(12, 30)
+
+    """
+    return datetime_to_date_and_time(dt)[1]
+
+
+class TimeUnit(enum.IntEnum):
+    """An enum to represent units with which we can compute deltas."""
+
+    MONDAYS = 0
+    TUESDAYS = 1
+    WEDNESDAYS = 2
+    THURSDAYS = 3
+    FRIDAYS = 4
+    SATURDAYS = 5
+    SUNDAYS = 6
+    SECONDS = 7
+    MINUTES = 8
+    HOURS = 9
+    DAYS = 10
+    WORKDAYS = 11
+    WEEKS = 12
+    MONTHS = 13
+    YEARS = 14
+
+    @classmethod
+    def is_valid(cls, value: Any):
+        if isinstance(value, int):
+            return cls(value) is not None
+        elif isinstance(value, TimeUnit):
+            return cls(value.value) is not None
+        elif isinstance(value, str):
+            return cls.__members__[value] is not None
+        else:
+            print(type(value))
+            return False
+
+
+def n_timeunits_from_base(count: int, unit: TimeUnit, base: datetime.datetime) -> datetime.datetime:
+    """Return a datetime that is N units before/after a base datetime.
+    e.g.  3 Wednesdays from base datetime, 2 weeks from base date, 10
+    years before base datetime, 13 minutes after base datetime, etc...
+    Note: to indicate before/after the base date, use a positive or
+    negative count.
+
+    >>> base = string_to_datetime("2021/09/10 11:24:51AM-0700")[0]
+
+    The next (1) Monday from the base datetime:
+    >>> n_timeunits_from_base(+1, TimeUnit.MONDAYS, base)
+    datetime.datetime(2021, 9, 13, 11, 24, 51, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))
+
+    Ten (10) years after the base datetime:
+    >>> n_timeunits_from_base(10, TimeUnit.YEARS, base)
+    datetime.datetime(2031, 9, 10, 11, 24, 51, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))
+
+    Fifty (50) working days (M..F, not counting holidays) after base datetime:
+    >>> n_timeunits_from_base(50, TimeUnit.WORKDAYS, base)
+    datetime.datetime(2021, 11, 23, 11, 24, 51, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))
+
+    Fifty (50) days (including weekends and holidays) after base datetime:
+    >>> n_timeunits_from_base(50, TimeUnit.DAYS, base)
+    datetime.datetime(2021, 10, 30, 11, 24, 51, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))
+
+    Fifty (50) months before (note negative count) base datetime:
+    >>> n_timeunits_from_base(-50, TimeUnit.MONTHS, base)
+    datetime.datetime(2017, 7, 10, 11, 24, 51, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))
+
+    Fifty (50) hours after base datetime:
+    >>> n_timeunits_from_base(50, TimeUnit.HOURS, base)
+    datetime.datetime(2021, 9, 12, 13, 24, 51, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))
+
+    Fifty (50) minutes before base datetime:
+    >>> n_timeunits_from_base(-50, TimeUnit.MINUTES, base)
+    datetime.datetime(2021, 9, 10, 10, 34, 51, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))
+
+    Fifty (50) seconds from base datetime:
+    >>> n_timeunits_from_base(50, TimeUnit.SECONDS, base)
+    datetime.datetime(2021, 9, 10, 11, 25, 41, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))
+
+    Next month corner case -- it will try to make Feb 31, 2022 then count
+    backwards.
+    >>> base = string_to_datetime("2022/01/31 11:24:51AM-0700")[0]
+    >>> n_timeunits_from_base(1, TimeUnit.MONTHS, base)
+    datetime.datetime(2022, 2, 28, 11, 24, 51, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))
+
+    Last month with the same corner case
+    >>> base = string_to_datetime("2022/03/31 11:24:51AM-0700")[0]
+    >>> n_timeunits_from_base(-1, TimeUnit.MONTHS, base)
+    datetime.datetime(2022, 2, 28, 11, 24, 51, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))
+
+    """
+    assert TimeUnit.is_valid(unit)
+    if count == 0:
+        return base
+
+    # N days from base
+    if unit == TimeUnit.DAYS:
+        timedelta = datetime.timedelta(days=count)
+        return base + timedelta
+
+    # N hours from base
+    elif unit == TimeUnit.HOURS:
+        timedelta = datetime.timedelta(hours=count)
+        return base + timedelta
+
+    # N minutes from base
+    elif unit == TimeUnit.MINUTES:
+        timedelta = datetime.timedelta(minutes=count)
+        return base + timedelta
+
+    # N seconds from base
+    elif unit == TimeUnit.SECONDS:
+        timedelta = datetime.timedelta(seconds=count)
+        return base + timedelta
+
+    # N workdays from base
+    elif unit == TimeUnit.WORKDAYS:
+        if count < 0:
+            count = abs(count)
+            timedelta = datetime.timedelta(days=-1)
+        else:
+            timedelta = datetime.timedelta(days=1)
+        skips = holidays.US(years=base.year).keys()
+        while count > 0:
+            old_year = base.year
+            base += timedelta
+            if base.year != old_year:
+                skips = holidays.US(years=base.year).keys()
+            if base.weekday() < 5 and datetime.date(base.year, base.month, base.day) not in skips:
+                count -= 1
+        return base
+
+    # N weeks from base
+    elif unit == TimeUnit.WEEKS:
+        timedelta = datetime.timedelta(weeks=count)
+        base = base + timedelta
+        return base
+
+    # N months from base
+    elif unit == TimeUnit.MONTHS:
+        month_term = count % 12
+        year_term = count // 12
+        new_month = base.month + month_term
+        if new_month > 12:
+            new_month %= 12
+            year_term += 1
+        new_year = base.year + year_term
+        day = base.day
+        while True:
+            try:
+                ret = datetime.datetime(
+                    new_year,
+                    new_month,
+                    day,
+                    base.hour,
+                    base.minute,
+                    base.second,
+                    base.microsecond,
+                    base.tzinfo,
+                )
+                break
+            except ValueError:
+                day -= 1
+        return ret
+
+    # N years from base
+    elif unit == TimeUnit.YEARS:
+        new_year = base.year + count
+        return datetime.datetime(
+            new_year,
+            base.month,
+            base.day,
+            base.hour,
+            base.minute,
+            base.second,
+            base.microsecond,
+            base.tzinfo,
+        )
+
+    if unit not in set(
+        [
+            TimeUnit.MONDAYS,
+            TimeUnit.TUESDAYS,
+            TimeUnit.WEDNESDAYS,
+            TimeUnit.THURSDAYS,
+            TimeUnit.FRIDAYS,
+            TimeUnit.SATURDAYS,
+            TimeUnit.SUNDAYS,
+        ]
+    ):
+        raise ValueError(unit)
+
+    # N weekdays from base (e.g. 4 wednesdays from today)
+    direction = 1 if count > 0 else -1
+    count = abs(count)
+    timedelta = datetime.timedelta(days=direction)
+    start = base
+    while True:
+        dow = base.weekday()
+        if dow == unit.value and start != base:
+            count -= 1
+            if count == 0:
+                return base
+        base = base + timedelta
+
+
+def get_format_string(
     *,
     date_time_separator=" ",
     include_timezone=True,
     include_dayname=False,
+    use_month_abbrevs=False,
     include_seconds=True,
     include_fractional=False,
     twelve_hour=True,
 ) -> str:
-    """A nice way to convert a datetime into a string."""
+    """
+    Helper to return a format string without looking up the documentation
+    for strftime.
+
+    >>> get_format_string()
+    '%Y/%m/%d %I:%M:%S%p%z'
+
+    >>> get_format_string(date_time_separator='@')
+    '%Y/%m/%d@%I:%M:%S%p%z'
+
+    >>> get_format_string(include_dayname=True)
+    '%a/%Y/%m/%d %I:%M:%S%p%z'
+
+    >>> get_format_string(include_dayname=True, twelve_hour=False)
+    '%a/%Y/%m/%d %H:%M:%S%z'
+
+    """
     fstring = ""
     if include_dayname:
         fstring += "%a/"
-    fstring = f"%Y/%b/%d{date_time_separator}"
+
+    if use_month_abbrevs:
+        fstring = f"{fstring}%Y/%b/%d{date_time_separator}"
+    else:
+        fstring = f"{fstring}%Y/%m/%d{date_time_separator}"
     if twelve_hour:
         fstring += "%I:%M"
         if include_seconds:
@@ -50,11 +566,83 @@ def datetime_to_string(
         fstring += ".%f"
     if include_timezone:
         fstring += "%z"
+    return fstring
+
+
+def datetime_to_string(
+    dt: datetime.datetime,
+    *,
+    date_time_separator=" ",
+    include_timezone=True,
+    include_dayname=False,
+    use_month_abbrevs=False,
+    include_seconds=True,
+    include_fractional=False,
+    twelve_hour=True,
+) -> str:
+    """
+    A nice way to convert a datetime into a string; arguably better than
+    just printing it and relying on it __repr__().
+
+    >>> d = string_to_datetime(
+    ...                        "2021/09/10 11:24:51AM-0700",
+    ...                       )[0]
+    >>> d
+    datetime.datetime(2021, 9, 10, 11, 24, 51, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))
+    >>> datetime_to_string(d)
+    '2021/09/10 11:24:51AM-0700'
+    >>> datetime_to_string(d, include_dayname=True, include_seconds=False)
+    'Fri/2021/09/10 11:24AM-0700'
+
+    """
+    fstring = get_format_string(
+        date_time_separator=date_time_separator,
+        include_timezone=include_timezone,
+        include_dayname=include_dayname,
+        use_month_abbrevs=use_month_abbrevs,
+        include_seconds=include_seconds,
+        include_fractional=include_fractional,
+        twelve_hour=twelve_hour,
+    )
     return dt.strftime(fstring).strip()
 
 
+def string_to_datetime(
+    txt: str,
+    *,
+    date_time_separator=" ",
+    include_timezone=True,
+    include_dayname=False,
+    use_month_abbrevs=False,
+    include_seconds=True,
+    include_fractional=False,
+    twelve_hour=True,
+) -> Tuple[datetime.datetime, str]:
+    """A nice way to convert a string into a datetime.  Returns both the
+    datetime and the format string used to parse it.  Also consider
+    dateparse.dateparse_utils for a full parser alternative.
+
+    >>> d = string_to_datetime(
+    ...                        "2021/09/10 11:24:51AM-0700",
+    ...                       )
+    >>> d
+    (datetime.datetime(2021, 9, 10, 11, 24, 51, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200))), '%Y/%m/%d %I:%M:%S%p%z')
+
+    """
+    fstring = get_format_string(
+        date_time_separator=date_time_separator,
+        include_timezone=include_timezone,
+        include_dayname=include_dayname,
+        use_month_abbrevs=use_month_abbrevs,
+        include_seconds=include_seconds,
+        include_fractional=include_fractional,
+        twelve_hour=twelve_hour,
+    )
+    return (datetime.datetime.strptime(txt, fstring), fstring)
+
+
 def timestamp() -> str:
-    """Return a timestamp for now in Pacific timezone."""
+    """Return a timestamp for right now in Pacific timezone."""
     ts = datetime.datetime.now(tz=pytz.timezone("US/Pacific"))
     return datetime_to_string(ts, include_timezone=True)
 
@@ -67,7 +655,25 @@ def time_to_string(
     include_timezone=False,
     twelve_hour=True,
 ) -> str:
-    """A nice way to convert a datetime into a time (only) string."""
+    """A nice way to convert a datetime into a time (only) string.
+    This ignores the date part of the datetime.
+
+    >>> d = string_to_datetime(
+    ...                        "2021/09/10 11:24:51AM-0700",
+    ...                       )[0]
+    >>> d
+    datetime.datetime(2021, 9, 10, 11, 24, 51, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))
+
+    >>> time_to_string(d)
+    '11:24:51AM'
+
+    >>> time_to_string(d, include_seconds=False)
+    '11:24AM'
+
+    >>> time_to_string(d, include_seconds=False, include_timezone=True)
+    '11:24AM-0700'
+
+    """
     fstring = ""
     if twelve_hour:
         fstring += "%l:%M"
@@ -94,17 +700,63 @@ MinuteOfDay = NewType("MinuteOfDay", int)
 
 
 def minute_number(hour: int, minute: int) -> MinuteOfDay:
-    """Convert hour:minute into minute number from start of day."""
+    """
+    Convert hour:minute into minute number from start of day.
+
+    >>> minute_number(0, 0)
+    0
+
+    >>> minute_number(9, 15)
+    555
+
+    >>> minute_number(23, 59)
+    1439
+
+    """
     return MinuteOfDay(hour * 60 + minute)
 
 
 def datetime_to_minute_number(dt: datetime.datetime) -> MinuteOfDay:
-    """Convert a datetime into a minute number (of the day)"""
+    """
+    Convert a datetime into a minute number (of the day).  Note that
+    this ignores the date part of the datetime and only uses the time
+    part.
+
+    >>> d = string_to_datetime(
+    ...                        "2021/09/10 11:24:51AM-0700",
+    ...                       )[0]
+
+    >>> datetime_to_minute_number(d)
+    684
+
+    """
     return minute_number(dt.hour, dt.minute)
 
 
+def time_to_minute_number(t: datetime.time) -> MinuteOfDay:
+    """
+    Convert a datetime.time into a minute number.
+
+    >>> t = datetime.time(5, 15)
+    >>> time_to_minute_number(t)
+    315
+
+    """
+    return minute_number(t.hour, t.minute)
+
+
 def minute_number_to_time_string(minute_num: MinuteOfDay) -> str:
-    """Convert minute number from start of day into hour:minute am/pm string."""
+    """
+    Convert minute number from start of day into hour:minute am/pm
+    string.
+
+    >>> minute_number_to_time_string(315)
+    ' 5:15a'
+
+    >>> minute_number_to_time_string(684)
+    '11:24a'
+
+    """
     hour = minute_num // 60
     minute = minute_num % 60
     ampm = "a"
@@ -119,7 +771,24 @@ def minute_number_to_time_string(minute_num: MinuteOfDay) -> str:
 
 
 def parse_duration(duration: str) -> int:
-    """Parse a duration in string form."""
+    """
+    Parse a duration in string form into a delta seconds.
+
+    >>> parse_duration('15 days, 2 hours')
+    1303200
+
+    >>> parse_duration('15d 2h')
+    1303200
+
+    >>> parse_duration('100s')
+    100
+
+    >>> parse_duration('3min 2sec')
+    182
+
+    """
+    if duration.isdigit():
+        return int(duration)
     seconds = 0
     m = re.search(r'(\d+) *d[ays]*', duration)
     if m is not None:
@@ -136,9 +805,24 @@ def parse_duration(duration: str) -> int:
     return seconds
 
 
-def describe_duration(age: int) -> str:
-    """Describe a duration."""
-    days = divmod(age, constants.SECONDS_PER_DAY)
+def describe_duration(seconds: int, *, include_seconds=False) -> str:
+    """
+    Describe a duration represented as a count of seconds nicely.
+
+    >>> describe_duration(182)
+    '3 minutes'
+
+    >>> describe_duration(182, include_seconds=True)
+    '3 minutes, and 2 seconds'
+
+    >>> describe_duration(100, include_seconds=True)
+    '1 minute, and 40 seconds'
+
+    describe_duration(1303200)
+    '15 days, 2 hours'
+
+    """
+    days = divmod(seconds, constants.SECONDS_PER_DAY)
     hours = divmod(days[1], constants.SECONDS_PER_HOUR)
     minutes = divmod(hours[1], constants.SECONDS_PER_MINUTE)
 
@@ -147,29 +831,90 @@ def describe_duration(age: int) -> str:
         descr = f"{int(days[0])} days, "
     elif days[0] == 1:
         descr = "1 day, "
+
     if hours[0] > 1:
         descr = descr + f"{int(hours[0])} hours, "
     elif hours[0] == 1:
         descr = descr + "1 hour, "
-    if len(descr) > 0:
+
+    if not include_seconds and len(descr) > 0:
         descr = descr + "and "
+
     if minutes[0] == 1:
         descr = descr + "1 minute"
     else:
         descr = descr + f"{int(minutes[0])} minutes"
+
+    if include_seconds:
+        descr = descr + ', '
+        if len(descr) > 0:
+            descr = descr + 'and '
+        s = minutes[1]
+        if s == 1:
+            descr = descr + '1 second'
+        else:
+            descr = descr + f'{s} seconds'
     return descr
 
 
-def describe_duration_briefly(age: int) -> str:
-    """Describe a duration briefly."""
-    days = divmod(age, constants.SECONDS_PER_DAY)
+def describe_timedelta(delta: datetime.timedelta) -> str:
+    """
+    Describe a duration represented by a timedelta object.
+
+    >>> d = datetime.timedelta(1, 600)
+    >>> describe_timedelta(d)
+    '1 day, and 10 minutes'
+
+    """
+    return describe_duration(int(delta.total_seconds()))  # Note: drops milliseconds
+
+
+def describe_duration_briefly(seconds: int, *, include_seconds=False) -> str:
+    """
+    Describe a duration briefly.
+
+    >>> describe_duration_briefly(182)
+    '3m'
+
+    >>> describe_duration_briefly(182, include_seconds=True)
+    '3m 2s'
+
+    >>> describe_duration_briefly(100, include_seconds=True)
+    '1m 40s'
+
+    describe_duration_briefly(1303200)
+    '15d 2h'
+
+    """
+    days = divmod(seconds, constants.SECONDS_PER_DAY)
     hours = divmod(days[1], constants.SECONDS_PER_HOUR)
     minutes = divmod(hours[1], constants.SECONDS_PER_MINUTE)
 
-    descr = ""
+    descr = ''
     if days[0] > 0:
-        descr = f"{int(days[0])}d "
+        descr = f'{int(days[0])}d '
     if hours[0] > 0:
-        descr = descr + f"{int(hours[0])}h "
-    descr = descr + f"{int(minutes[0])}m"
-    return descr
+        descr = descr + f'{int(hours[0])}h '
+    if minutes[0] > 0 or (len(descr) == 0 and not include_seconds):
+        descr = descr + f'{int(minutes[0])}m '
+    if minutes[1] > 0 and include_seconds:
+        descr = descr + f'{int(minutes[1])}s'
+    return descr.strip()
+
+
+def describe_timedelta_briefly(delta: datetime.timedelta) -> str:
+    """
+    Describe a duration represented by a timedelta object.
+
+    >>> d = datetime.timedelta(1, 600)
+    >>> describe_timedelta_briefly(d)
+    '1d 10m'
+
+    """
+    return describe_duration_briefly(int(delta.total_seconds()))  # Note: drops milliseconds
+
+
+if __name__ == '__main__':
+    import doctest
+
+    doctest.testmod()