Declination varies a little from year to year. Just look at an old Topo of your
location and compare it to today's or to your GPS. Some airport runways have to
have their numerals re-painted when they are no longer correct, and those are 10
degrees wide.

Delorme GPS's allow one to upload a declination correction file.
Like all GPS's there is a database, or a table, of offsets based on position.
Perhaps like the Ionospheric Grids of WAAS, a large square where ones magnetic
declination is the same as everywhere else in that grid.

I've looked at Delorme's file, it's binary. Unlike Magellan's firmware which
I've been able to peek inside, I can't make heads or tails of what Delorme is
doing.

I also do not know if Garmin makes a correction file available.