UTCDateTime.__sub__ returns only TimeDelta without Date-Delta

[ArneBachmannDLR]

I was expecting it to return a DateTimeDelta, but it is:

    def __sub__(self, other: _AwareDateTime) -> TimeDelta: ...

Hint: I want to compute (dt - UTCDateTime(1970, 1, 1)).days.

Update: this seems to work: (dt._py_dt - datetime.datetime(1970, 1, 1, tzinfo=datetime.UTC)).days

[ariebovenberg]

Hi @ArneBachmannDLR, thanks for posting this issue—this is indeed a part of the API I'd like to see improved myself as well.

Before I go into detail about potential solutions, here are two workarounds, with different semantics:

1. if you care about calendar days, do your arithmetic on Date objects:

   >>> dt.date() - Date(1970, 1, 1)
   DateDelta(P54Y3M8D)
   # note that this may be different, depending on `dt` UTC offset!
   >>> dt.as_utc().date() - Date(1970, 1, 1)
   DateDelta(P54Y3M9D)

2. If you care about days as exact 24-hour time units, make this explicit:

   >>> (dt - UTCDateTime(1970, 1, 1)).in_hours() / 24
   19822.5

   the reason there is no in_days() is because days are sometimes more or less than 24 hours long, due to daylight saving time.

A note on your workaround

• Please use the public .py_datetime() instead of the private _py_dt. The private attribute may be removed in later releases.
• This solution is equivalent to workaround nr. 2 above

A long-term solution

The complicating factor here is that if UTCDateTime - x simply returns a DateTimeDelta, that you can't do .in_hours() or similar since calendar units aren't exact.

The solution is perhaps some kind of Difference object which can be interpreted on the calendar, as well as the timeline. I will create an issue about this in the future, and link it here.

[ArneBachmannDLR]

Just noticed that

• (UTCDateTime(2022, 2, 11) - UTCDateTime(1970, 1, 1)).days gives 10 instead of 19358
• (dt - UTCDateTime(1970, 1, 1)).in_hours() // 24 returns a float instead of an int

[ariebovenberg]

Indeed:

• .days refers to the days component of the DateDelta: Date(2024, 4, 11) - Date(1970, 1, 1) is DateDelta(P54Y3M10D), i.e. .years=54, .months=3, .days=10. The .days attribute is perhaps too ambiguous, and can lead to misunderstandings—especially coming from datetime.timedelta. Perhaps renaming it to days_component is less ambiguous 🤔
• returning a float is actually correct right? There probably isn't an exact 24h-divisible difference between two moments. You need to explicitly decide whether to round up or down

edit: mistaken first conclusion