It isn't really a simple modification. Due to how the goal log works and other little areas it isn't a simple thing to change. Bad early design really. You can look at Merc_DonationManager_Model_Donation::updateGoalInfo() in library/Merc/DonationManager/Model/Donation.php on line 281 and change it to check for the current year instead. I don't know if this will work as intended though. I might add in a hacky way of doing it so people can have one of a few presets for now (1 week, 2 weeks, 1 month, 3 months, 6 months, 1 year, 2 years sounds ok to me). Then later on when I have time again do it properly with custom set times and functionality for when a goal ends.
Honestly, even just having the 1 week, 2 weeks, 1 month, 3 months, 6 months, 1 year, 2 years, as you have said, would make a world of difference. Thanks for the quick reply.