So if two sets of events that can't have occurred simultaneously bear the same stardate, wouldn't this seem to indicate that TOS stardates are relative and not absolute, as Roddenberry himself suggested?
Either that - or then our heroes really lived the same days twice.
Which is apparently exactly what happens in the aftermath of "The Naked Time". And after "Tomorrow is Yesterday", time travel appears to become a routine mission type for our heroes... Perhaps they just decided to get more lightyearage out of their five-year mission?
Does anybody know how I can find out what the stardates for these episodes of TOS:
You mean made-up stardates? Does it matter who made them up?
1) The Omega Glory: This adaptation of a pilot/show pitch script features no plot points that would fix the story in time. Since it's one of the Chekov-less episodes, we could place it very early in the series even if we believe that Chekov only came aboard in "Catspaw". Really, we could even place it before "Miri" to explain how our heroes are not particularly impressed by the existence of a duplicate Earth there - they have already seen weirder.
2) Assignment Earth: Rather irrelevant, since no part of the episode takes place in the era where stardates are in use. It suffices that this happens after "Tomorrow is Yesterday" where our heroes learned the time travel technique, thus after SD 3120. How long Starfleet mulled it over before deciding to utilize the technique is anyone's guess.
3) Mirror, Mirror: The novel
Q Squared suggests SD 3823.7; the earlier novel
Dark Mirror said 4428.9, but we could argue this was a Mirror date.

Both are essentially okay.
4) Day of the Dove: Dialogue indicates this is three years after "Errand of Mercy", so if we believe that a thousand stardates mark a year, then anything between 6200 and 7200 would be fine, and anything above 5700 would be acceptable. (Unless those were Klingon years, that is!)
This episode would then probably be one of the very last in TOS, if not the last. And it appears that nothing in the plot goes against this.
FWIW, Memory Beta lists SD 3372.7 without any explanation. Perhaps this is mentioned in some novel referring to the events, such as the
War Stories anthology which features the return of the creature from the TOS episode?
That Which Survives: Nothing solid there, but
Mr Scott's Guide to the Enterprise suggests it happened on SD 5978.2. Yet a closely related novel
Gateways: One Small Step is mum on the subject of dates.
Timo Saloniemi