## On the Differences of Months

July 28, 2011 § Leave a comment

Dealing with dates is hard. Dealing date calculations is harder. Luckily, in the Rails world we have ActiveSupport to help us with a lot of this. It actually does so much, that I usually forget how much of a pain dealing with dates is suppose to be. However, there are times now and then that I am reminded. Little edge cases that haven’t yet been built into ActiveSupport. One such edge case is determining the number of months between two dates. Why do months have to have different numbers of days?

This particular use case can be very useful when dealing with recurring payments. Calculating the number of payment cycles a subscriber has gone through can tell you how much revenue they have generated. I have put together two class methods for the Time object that will calculate just this. One is a simple loop that takes time proportional to the time between the start and end times, and the other a more efficient direct calculation of the same number.

The simple loop looks like:

def months_between2(start_date, end_date) return -months_between2(end_date, start_date) if end_date < start_date count = 1 while true return count - 1 if (start_date + count.months) > end_date count += 1 end end

It just starts at the `start_date`

and keeps adding months until it passes the `end_date`

. Not particularly difficult, but should get the job done for most cases. All the complexity of the more efficient version comes from checks dealing with the various cases arising from different months having a different number of days. Anyways, the source code for that one looks much nicer over on GitHub.

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