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Improving time_length for 29 Feb (when dealing with date time) #295

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merged 1 commit into from
Jan 4, 2015

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larmarange
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With previous version, we had:

> naiss <- ymd_hms("1992-02-29 12:00:00")
> evt <- ymd_hms("2014-03-01 01:00:00")
> time_length(interval(naiss, evt), "years")
[1] 21.99874

We should consider that the anniversary occurs between 28 Feb at 23:59:59 and 1st March at 00:00:00. Therefore, %m++% should rollback at 00:00:00. In that case:

> time_length(interval(naiss, evt), "years")
[1] 22.00011

@larmarange larmarange changed the title Impriving time_length for 29 Feb (when dealing with date time) Improving time_length for 29 Feb (when dealing with date time) Jan 1, 2015
@larmarange
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A quick comment about period computation from intervals:

> naiss <- ymd_hms("1992-02-29 12:00:00")
> evt <- ymd_hms("2014-03-01 01:00:00")
> as.period(interval(naiss, evt))
[1] "22y 0m -1d 13H 0M 0S"

Should we avoid negative days?

@vspinu
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vspinu commented Jan 1, 2015

This should not happen. It's a bug in as.period.
On Jan 1, 2015 6:04 AM, "Joseph" notifications@github.com wrote:

A quick comment about period computation from intervals:

naiss <- ymd_hms("1992-02-29 12:00:00")> evt <- ymd_hms("2014-03-01 01:00:00")> as.period(interval(naiss, evt))
[1] "22y 0m -1d 13H 0M 0S"

Should we avoid negative days?


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#295 (comment).

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vspinu commented Jan 2, 2015

naiss <- ymd_hms("1992-02-29 12:00:00")
evt <- ymd_hms("2014-03-01 01:00:00")
time_length(interval(naiss, evt), "years")
[1] 21.99874

Well. There is a mirror problem with negative intervals that use %m+%:

> time_length(interval(ymd_hms('2000-02-29 12:00:00'), ymd_hms('1999-02-28 20:00:00')), "years")
[1] -0.9990868

So we will have to change %m+% as well, and I am not sure that's a good idea. %m+% will basically make all intraday timestamps for Feb 29 equal, and that's clearly not suitable for intraday analysis.

One easy solution is to add an optional argument to rollback and .month_plus and .month_plus_plus. Something like preserve_hms = TRUE.

BTW. your .rollback_day_one can also be integrated into rallback with an optional argument roll_to_first = FALSE or something similar.

@larmarange
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You are right.

We can also add roll_to_first to .month_plus. In that scenario, we don't need %m++ and .month_plus_plus (as it's not exported) and we can use directly .month_plus in time_length.

I will update the pull request.

@larmarange
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When we think with time, even for negative intervals we should not use %m-%. In fact, the anniversary should be the 28 Feb at 24:00:00, i.e. the 1st March at 00:00:00.

Therefore, the code is simpler.

Now, we obtain:

> time_length(interval(ymd_hms('2000-02-29 12:00:00'), ymd_hms('1999-02-28 20:00:00')), "years")
[1] -1.000457

@larmarange larmarange force-pushed the master branch 3 times, most recently from 680d4a7 to f1c22f3 Compare January 4, 2015 13:29
Two new arguments (roll_to_first and preserve_hms) for rollback and
.month_plus.  time_length for intervals updated. %m++% not needed
anymore.
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Tests and examples added for rollback. Code of time_length updated with ifelse(x@.Data < 0, -1, 1)

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vspinu commented Jan 4, 2015

Great! Thanks.

vspinu added a commit that referenced this pull request Jan 4, 2015
Improving time_length for 29 Feb (when dealing with date time)
@vspinu vspinu merged commit 215c91a into tidyverse:master Jan 4, 2015
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2 participants