When Gene Roddenberry came up with Stardates, he had this to say (bolding mine):
In answering these questions, I came up with the statement that "this time system adjusts for shifts in relative time which occur due to the vessel's speed and space warp capability. It has little relationship to Earth's time as we know it. One hour aboard the U.S.S. Enterprise at different times may equal as little as three Earth hours. The star dates specified in the log entry must be computed against the speed of the vessel, the space warp, and its position within our galaxy, in order to give a meaningful reading." Therefore star date would be one thing at one point in the galaxy and something else again at another point in the galaxy.
This is actually sort of similar to how we track time here on Earth. If I go across the time zones quickly enough, I can end up in the "past" or the "future" according to the date/time. The part I bolded implies that there's some sort of similar natural breakdown you can make on the galaxy itself (which, presumably, is only known to spacefaring cultures within Star Trek).
Now, if instead of a 24-hour day, think about if we used percentages: Midnight is 0.0, noon is 0.5, 6pm is 0.75, etc. The concept of percentages is easily translatable across cultures, but whether 0.0 occurs at sunrise, sunset, or (in our case) sometime else entirely is culture-dependent.
Likewise, we can't really know when Stardate 12345.44 is, unless there are other things we know. If one culture says "now" is that Stardate, and we're currently on Stardate 23456.67, then it's just like someone from the US talking to someone in France. The difference between ".44" and ".67" would be equivalent to the difference between times in different time zones. The difference between the "12345" and "23456" parts is just the difference between calendars that started at different points.
Now, there's another reallife factor we can add in: Email, which automatically includes a "Sent at" date. If Degra's message had both an automatic "sent at" date and the rendezvous date in the message, then it would be possible (but not simple) to use those galactic conditions to calculate the time from "now" to "then", and then convert to human time spans.
Note that I'm not really saying they were ever synchronized across cultures, but that it's possible for T'Pol to convert between them despite the Vulcans not having had prior contact with the Xindi.