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At first glance the contract of the beautiful 1.month.ago construct in Rails is maybe obvious. However, what should it return if the current time is the 31st of March? Remember that february only has 28 days. Well, according to Rails 1.2.3 the answer is 1st of March, but according to Edge Rails the answer is (and I agree with Edge Rails here) 28:th of February.
The 1.month.ago issue is something that bit me and that you should be aware of. So with Rails 1.2.3, if you want to be sure to get the beginning of the previous month, you need to say 1.month.until(Time.now.beginning_of_month). At least that's the workaround I'm using now.