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I noticed that 29030400 is used very often in the Expires directives for static files.

Google recommends to cache this kind of files up to 1 year (1 month at least).

I did the math: 29030400 seconds = 336 days

That is about 1 year minus 1 month, so it perfectly falls in the recommended interval, but the question is why exactly 29030400? And not 31536000 seconds = 365 days for example? Just a blind copy/paste of a value that was set randomly in the old days? Or does it have another explanation?

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Blind copy/paste! –  Pazis Nov 24 '13 at 14:50
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1 Answer

up vote 15 down vote accepted

It might come from

60 * 60 * 24 * 7 * 4 * 12 = 29030400

where each month consists of exactly 4 weeks.

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Great answer m8 :-) many people forget that 4 weeks isn't a month. –  bybe Nov 25 '13 at 0:32
    
makes sense, genious, accepted. –  heytools Nov 25 '13 at 12:43
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