A few weeks ago I was working on getting the beginning and end of the week for any given DateTime.
This would've been easy if I realized that the AddDays function of the .NET DateTime actually returned a new DateTime.
I spent a full morning pragmatically working out which month/day/date it was and when.
Once I realized this I re-wrote my code in about 1 minute. Only slowed down by typing speed.
Anyway I hope this is something that you all find useful to know.