# UTCDateTime and leap seconds?

**URL:** <https://discourse.obspy.org/t/utcdatetime-and-leap-seconds/624>\
**Category:** Users Mailing List Archive\
**Tags:** archive\
**Created:** [March 17, 2017, 2:26pm UTC](https://discourse.obspy.org/t/utcdatetime-and-leap-seconds/624 "2017-03-17T14:26:11Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Christian\_Weidle1](https://discourse.obspy.org/letter_avatar_proxy/v4/letter/c/aeb1de/32.png) [@Christian\_Weidle1](https://discourse.obspy.org/u/Christian_Weidle1)\
**Post date:** [March 17, 2017, 2:26pm UTC](https://discourse.obspy.org/t/utcdatetime-and-leap-seconds/624/1 "2017-03-17T14:26:11Z")

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Hi,

as I ran across an issue on leap seconds elsewhere, I wondered if/how they are dealt with in the UTCDateTime object.

In [73]: t0 = UTCDateTime(2017,1,1)

In [74]: t0 -1  
Out[74]: 2016-12-31T23:59:59.000000Z

... while in principle t0-1 should be 2016-12-31T23:59:60.000000Z due to the recent leapsecond.  
With 27 leapseconds since 1972 this might be a problem in some use cases.

Has this been considered somewhere in the development?

Cheers  
Christian

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**Author:** ![LionKrischer](https://discourse.obspy.org/user_avatar/discourse.obspy.org/lionkrischer/32/219_2.png) [@LionKrischer](https://discourse.obspy.org/u/LionKrischer)\
**Post date:** [March 17, 2017, 2:52pm UTC](https://discourse.obspy.org/t/utcdatetime-and-leap-seconds/624/2 "2017-03-17T14:52:45Z")

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Hi Christian,

currently UTCDateTime is not aware of leap seconds. We have some discussions online and the current plan is to enable it for time difference calculations, e.g

time\_a - time\_b

If there is a leap-second in-between it will be reflected in that. Same when adding a time and a duration.

Your example contains a leap second time which we will likely not handle because neither our underlying time representation nor Python can actually deal with these “60-second” times (there were some internal discussions on the python dev mailing list but if I remember correctly they deemed the problem not serious enough to deal with it). We’d probably just raise an error in these cases.

It might be possible to add a flag “is\_leapsecond” to the UTCDateTime object and than gracefully handle these cases but this will likely bring along quite some additional complexity.

It is indeed a bit problematic but it only matters for time difference that have one or more leap seconds in between so the actual damage is likely limited. But we should somehow find a proper way to deal with it.

Please join the discussion on Github if you have some more ideas:

Cheers!

Lion