Ship it!

After 4 years of professional programming the most important thing that I learned is "user is stupid".
Once I modified old code that was summing salaries (I added extra column to the result), nothing else. My result was rejected because all salaries was empty. Period of data was from first day of the month from user selected date to user selected date. It turned out that user was selecting 27th day of the month (it was 27th then). I responded that salaries have full month period, and you'll have to choose end of the month... And then shitstorm began, that I messed up previous functionality. I tried to explain, but it wasn't working. It ends up with user selecting any date and I'm doing end-of-the-month in the background^^
It's my first rant, welcome to you all :)