How to find if a date fits an interval in either PHP or MySQL?

Problem :

Let's say I have a datetime, June 16 2011 at 7:00. I want to be able to check at, say, August 5 2011 at 7:00 and be able to tell that it is exactly a multiple of 1 day since the first date, whereas 7:01 would not count, since it is not an exact multiple.

Another test set: Let's say we have June 16 2011 at 7:00, and I want to check if a particular minute is within an interval of exactly 2 hours since then. So 9:00, 11:00, 13:00, etc. would count, but 9:30 and 10:00 would not. And this could continue for days and months - September 1 at 7:00 would still count as within every 2 hours. (And no, at the moment I don't know how I'm going to handle DST :D)

I thought about it for a moment and couldn't think of anything already existing in PHP or MySQL to do this easily but hell, it could, so I wanted to throw this up and ask before I start reinventing the wheel.

This is on PHP 5.1, sadly.

Solution :

select *
from test
where datetimefield > '2011-06-16 07:00:00'
mod(timestampdiff(second,'2011-06-16 07:00:00',datetimefield),7200) = 0

This example will give you all the records greater than '2011-06-16 07:00:00' where the field is exactly a multiple of 2 hours.

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