# Is it Time bug?

**URL:** <https://www.ruby-forum.com/t/is-it-time-bug/53239>\
**Category:** Ruby\
**Created:** [January 30, 2006, 7:47am UTC](https://www.ruby-forum.com/t/is-it-time-bug/53239 "2006-01-30T07:47:46Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![python](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ee7513/32.png) [@python](https://www.ruby-forum.com/u/python)\
**Post date:** [January 30, 2006, 7:47am UTC](https://www.ruby-forum.com/t/is-it-time-bug/53239/1 "2006-01-30T07:47:46Z")

</div>

Hi all! I’m writing program some days ago which calculates every  
Thursday day in interval given by 2 Time values and see interesting  
result described later. Is it my or ruby bug?

## Result

Thu Sep 01 00:00:00 MSD 2005  
Thu Sep 08 00:00:00 MSD 2005  
Thu Sep 15 00:00:00 MSD 2005  
Thu Sep 22 00:00:00 MSD 2005  
Thu Sep 29 00:00:00 MSD 2005  
Thu Oct 06 00:00:00 MSD 2005  
Thu Oct 13 00:00:00 MSD 2005  
Thu Oct 20 00:00:00 MSD 2005  
Thu Oct 27 00:00:00 MSD 2005  
Wed Nov 02 23:00:00 MSK 2005  
Wed Nov 09 23:00:00 MSK 2005  
Wed Nov 16 23:00:00 MSK 2005  
Wed Nov 23 23:00:00 MSK 2005  
Wed Nov 30 23:00:00 MSK 2005  
Wed Dec 07 23:00:00 MSK 2005

## Code

t1 = Time.mktime(2005, 9, 1)  
t2 = Time.mktime(2005, 12, 11)  
while t1.to\_i \< t2.to\_i  
p t1  
t1 += 3600_24_7  
end

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<div class="post-metadata">

**Author:** ![python](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ee7513/32.png) [@python](https://www.ruby-forum.com/u/python)\
**Post date:** [January 30, 2006, 7:55am UTC](https://www.ruby-forum.com/t/is-it-time-bug/53239/2 "2006-01-30T07:55:13Z")

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Valerij KIR wrote:

> Thu Sep 22 00:00:00 MSD 2005  
> Wed Dec 07 23:00:00 MSK 2005

This is what I get. Looks like it is the effect of the PDT-\>PST time  
change (or in your case MSD-\>MSK).

Thu Sep 01 00:00:00 Pacific Daylight Time 2005  
Thu Sep 08 00:00:00 Pacific Daylight Time 2005  
Thu Sep 15 00:00:00 Pacific Daylight Time 2005  
Thu Sep 22 00:00:00 Pacific Daylight Time 2005  
Thu Sep 29 00:00:00 Pacific Daylight Time 2005  
Thu Oct 06 00:00:00 Pacific Daylight Time 2005  
Thu Oct 13 00:00:00 Pacific Daylight Time 2005  
Thu Oct 20 00:00:00 Pacific Daylight Time 2005  
Thu Oct 27 00:00:00 Pacific Daylight Time 2005  
Wed Nov 02 23:00:00 Pacific Standard Time 2005  
Wed Nov 09 23:00:00 Pacific Standard Time 2005  
Wed Nov 16 23:00:00 Pacific Standard Time 2005  
Wed Nov 23 23:00:00 Pacific Standard Time 2005  
Wed Nov 30 23:00:00 Pacific Standard Time 2005  
Wed Dec 07 23:00:00 Pacific Standard Time 2005

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<div class="post-metadata">

**Author:** ![python](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ee7513/32.png) [@python](https://www.ruby-forum.com/u/python)\
**Post date:** [January 30, 2006, 8:13am UTC](https://www.ruby-forum.com/t/is-it-time-bug/53239/3 "2006-01-30T08:13:20Z")

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First of all for doing stuff like this I highly recommend checking out  
Runt [1], it’s a temporal expression library. Think regular  
expressions for time.

Anyway I think it has to do with the fact that that the Date lib  
accounts for leap years, but your code doesn’t. To explain it a bit…

There are 365.25 days in a year, not 365. This means that there are  
31557600 seconds in a year, rather than the 31536000 that your code  
would generate. That leaves you with 21600 unnoticed seconds. 365 /  
21600 = 0.0168981481, which is how many seconds are lost in each day.  
There are 63 days between your start date of Sep 01 and the “weird”  
date of Nov 02. 63 \* 0.0168981481 = 1.03, so there’s one lost second,  
causing the day to not roll over.

Pretty fun, huh? 🙂

Now that I’ve taken the time to identify the problem, I honestly don’t  
feel like solving it…but hopefully you can get started with this new  
found info 🙂 Again, I’d recommend checking out runt.

Pat

[1] [http://runt.rubyforge.org/](http://runt.rubyforge.org/)

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**Author:** ![python](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ee7513/32.png) [@python](https://www.ruby-forum.com/u/python)\
**Post date:** [January 30, 2006, 8:13am UTC](https://www.ruby-forum.com/t/is-it-time-bug/53239/4 "2006-01-30T08:13:20Z")

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On 1/29/06, Joel VanderWerf [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > Thu Sep 15 00:00:00 MSD 2005  
> > Wed Nov 30 23:00:00 MSK 2005  
> > end  
> > Thu Oct 06 00:00:00 Pacific Daylight Time 2005  
> > –  
> > vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Damn, you win. Thought I had found an interesting “gotcha” 🙂

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**Author:** ![python](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ee7513/32.png) [@python](https://www.ruby-forum.com/u/python)\
**Post date:** [January 30, 2006, 11:05am UTC](https://www.ruby-forum.com/t/is-it-time-bug/53239/5 "2006-01-30T11:05:42Z")

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Pat M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:

> 21600 = 0.0168981481, which is how many seconds are lost in each day.  
> There are 63 days between your start date of Sep 01 and the “weird”  
> date of Nov 02. 63 \* 0.0168981481 = 1.03, so there’s one lost second,  
> causing the day to not roll over.
> 
> Pretty fun, huh? 🙂

I don’t believe that this is the cause of the problem here. Changing  
the dates from 5/1/2005 to 7/11/2005 (just shifting everything back 4  
months) does not cause the same effect. This is a daylight savings time  
issue, not a seconds-per-year issue. In 2005, daylight savings time  
went into effect on April 3, 2005, and terminated on 30 October, 2005  
… at least in the USA.

t1 = Time.mktime(2005, 5, 1)  
t2 = Time.mktime(2005, 7, 11)  
while t1.to\_i \< t2.to\_i  
p t1  
t1 += 3600_24_7  
end

Sun May 01 00:00:00 EDT 2005  
Sun May 08 00:00:00 EDT 2005  
Sun May 15 00:00:00 EDT 2005  
Sun May 22 00:00:00 EDT 2005  
Sun May 29 00:00:00 EDT 2005  
Sun Jun 05 00:00:00 EDT 2005  
Sun Jun 12 00:00:00 EDT 2005  
Sun Jun 19 00:00:00 EDT 2005  
Sun Jun 26 00:00:00 EDT 2005  
Sun Jul 03 00:00:00 EDT 2005  
Sun Jul 10 00:00:00 EDT 2005

Every day has exactly 86400 seconds (except every few years when a  
leap-second is added/subtracted). The seconds-per-year issue is covered  
by leap years.

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**Author:** ![python](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ee7513/32.png) [@python](https://www.ruby-forum.com/u/python)\
**Post date:** [January 30, 2006, 11:26am UTC](https://www.ruby-forum.com/t/is-it-time-bug/53239/6 "2006-01-30T11:26:50Z")

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On 1/30/06, Lloyd Z. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > 31557600 seconds in a year, rather than the 31536000 that your code  
> > months) does not cause the same effect. This is a daylight savings time  
> > issue, not a seconds-per-year issue.

Yeah, I realized that. To quote my reply to Joel, who uncovered the  
real issue:  
“Damn, you win. Thought I had found an interesting ‘gotcha’ :)”

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**Author:** ![python](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/ee7513/32.png) [@python](https://www.ruby-forum.com/u/python)\
**Post date:** [January 30, 2006, 11:20am UTC](https://www.ruby-forum.com/t/is-it-time-bug/53239/7 "2006-01-30T11:20:45Z")

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Lloyd Z. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:

> > 31557600 seconds in a year, rather than the 31536000 that your code  
> > Every day has exactly 86400 seconds (except every few years when a  
> > leap-second is added/subtracted). The seconds-per-year issue is covered  
> > by leap years.

… except on the daylight savings time transitions, of course. In that  
year in the USA, April 3 had 82,800 seconds, and October 30 had 90,000  
seconds, which accounts for the discrepancy in the original date range.

Also, the seconds-lost-per-day calculation above isn’t correct. You  
need to divide 21600 by 365, not the other way around. This yields a  
bit less than one minute per day.