Normally a compass that is surrounded by metal during storage will be fine; it won’t be affected in anyway. Store it next to metal or electronic equipment you will have a problem in that it could affect its accuracy.

If you can find a location with a known magnetic bearing on a particular date you do a sum to determine the magnet variation for the present date. Mark this new bearing on something solid. Set the compass on top of this and see how many degrees or mils the compass is out. When you come to use the compass just add or minus the number of degrees the compass was out and you have your true magnetic bearing.

These days it is made a lot easier by the fact there are programs that will give you the exact bearing for your specific location on that day.

I believe modern high-end gps units have a compass calibration algorithm built into them. But, I am sure you could quite easily work out how to do it on the standard cheapo gps units that are often talked about on the forum.