Game crashes every time I try to go back beyond the year 900.

Although this is annoying, it does not affect game play. There is no crash when the correct year is gone to.

The game doesn't actually crash, or at least I don't think it does. What probably happens is that you died in a way or place where the Dungeon Master can't or won't resurrect you. Most Infocom games would give you a restart/restore/quit prompt at that point, but Zork III quits to the operating system instead.

If you had run the game under DOS you'd probably be at the DOS prompt, but you'd still see the final text that was printed. The way GOG set things up, DOSBox itself quits.

One workaround would be to edit the [autoexec] section of dosboxZork3_single.conf to something like this:

[autoexec]
# Lines in this section will be run at startup.
mount C ".."
c:
cls
_zork3.com
@pause
cls
exit

(I've marked the added line with bold text.)

Even when the correct year is gone to, the following rare maybe 1 in 40 chance random game-ending death may or may not have happened to you: You go back to 776 GUE and do everything you're supposed to and this happens: "A particularly vicious-looking guard enters the room and eyes you. He grinds his teeth in a most unpleasant way, pulls a waffle out of his garment, and vaporizes you with a flick of his finger." You have died, game over. It forces you to save the game before going back in time, even when using the undo-allowing Frotz. This is totally unfair and Infocom never should have put this in.
Update: Good news. In the Obsessively Complete Infocom Catalog, there is a revision 25 of Zork III: zork3-r25-s860811.z3. This version ends with the restart/restore/quit prompt, so even if that unfair random death occurs, it's now possible to undo and recover from the waffle death, or restore to a recent save, while still in the game.
Spoiler alert:
Two bugs were removed in revision 25:
1. When you return from 776 to 948 GUE and return to the Jewel Room to retrieve all your stuff, Zork III no longer tries (and fails) to get the plaque when you "get all".
2. The "unlock cage with ring"... "(Taken). It doesn't seem to work." bug is gone. It was a useful workaround for the rare unfair death in earlier versions, but it's no longer needed.