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damiendr opened this issue Nov 11, 2017 · 5 comments
Open

datetime64("now") is undocumented #10003

damiendr opened this issue Nov 11, 2017 · 5 comments
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01 - Enhancement 04 - Documentation component: numpy.datetime64 (and timedelta64)

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@damiendr
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I found out that you can get the current UTC date with np.datetime64("now").

However this isn't documented here and there are a few things about the behaviour that aren't obvious. For instance it seems that the precision cannot be greater than one second:

In [95]: np.datetime64("now", "ms")
Out[95]: numpy.datetime64('2017-11-11T08:10:31.000')
@shoyer
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shoyer commented Nov 12, 2017

See here for the underlying C-function:

numpy/numpy/core/src/multiarray/datetime_strings.c

Lines 189 to 219 in 6582813

/*
* Parses (almost) standard ISO 8601 date strings. The differences are:
*
* + The date "20100312" is parsed as the year 20100312, not as
* equivalent to "2010-03-12". The '-' in the dates are not optional.
* + Only seconds may have a decimal point, with up to 18 digits after it
* (maximum attoseconds precision).
* + Either a 'T' as in ISO 8601 or a ' ' may be used to separate
* the date and the time. Both are treated equivalently.
* + Doesn't (yet) handle the "YYYY-DDD" or "YYYY-Www" formats.
* + Doesn't handle leap seconds (seconds value has 60 in these cases).
* + Doesn't handle 24:00:00 as synonym for midnight (00:00:00) tomorrow
* + Accepts special values "NaT" (not a time), "Today", (current
* day according to local time) and "Now" (current time in UTC).
*
* 'str' must be a NULL-terminated string, and 'len' must be its length.
* 'unit' should contain -1 if the unit is unknown, or the unit
* which will be used if it is.
* 'casting' controls how the detected unit from the string is allowed
* to be cast to the 'unit' parameter.
*
* 'out' gets filled with the parsed date-time.
* 'out_bestunit' gives a suggested unit based on the amount of
* resolution provided in the string, or -1 for NaT.
* 'out_special' gets set to 1 if the parsed time was 'today',
* 'now', or ''/'NaT'. For 'today', the unit recommended is
* 'D', for 'now', the unit recommended is 's', and for 'NaT'
* the unit recommended is 'Y'.
*
* Returns 0 on success, -1 on failure.
*/

Note that it also supports the string "today" indicating a local time.

If we're going to document it, we should probably make sure that the functionality makes sense. In the most recent version of datetime64, we aim for compatibility with Python's datetime module (without timezones). So it's a little strange that "now" means UTC rather than local time (see https://docs.python.org/2/library/datetime.html#datetime.datetime.now) Arguably, we should use something different like "utcnow" for UTC.

The precision issue could potentially be improved, possibly by switching to rely on Python's time.time() rather than our own implementation. For now it would indeed make sense to document the (potentially) limited precision.

@eric-wieser eric-wieser mentioned this issue Nov 28, 2017
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@mohdsanadzakirizvi
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Would like to work on this as my first contribution, is it open ?

@shoyer
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shoyer commented Dec 24, 2017

As noted above, I think we should probably clean up this functionality before documenting it, which would encourage more people to rely on functionality that would likely change.

@gosuto-inzasheru
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Yeah the precision thing is quite misleading I think, passing 'ms' only increases the size of the float.

@miccoli miccoli mentioned this issue Oct 31, 2021
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@Shreyjaradi
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I have raised an PR to add the example of np.datetime64("now", "ms")
#26477

@MattFisher MattFisher mentioned this issue Oct 20, 2024
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@eric-wieser @mattip @shoyer @damiendr @gosuto-inzasheru @mohdsanadzakirizvi @Shreyjaradi