Working with logic surrounding dates can sometimes be difficult and it's fairly common to come across really subtle date and time based bugs.
I was recently shown a bug in a PHP application that looks like it should be working at face value, but doesn't actually produce the correct result.
The issue in question was surrounding a date comparison. A collection of objects containing dates were compared in order to find those dates that were less than a month old. This worked fine in testing since all testing dates were less than a month old. After the system was in use for a number of months it became clear that the check wasn't working so further investigation was needed.
Read more here: https://www.hashbangcode.com/article/phpcsi-date-less-one-month-ago