A related one which I found is A + B - B != A In pretty much the same case, where A==some time and B == 1 month This came up in calculations for monthly subsc

A related one which I found is A + B - B != A In pretty much the same case, where A==some time and B == 1 month This came up in calculations for monthly subscriptions, trying to reach back in time to work out pro-rata charges for changing subscription details part way through a month.