I've checked than in my current fort I had goblins attack twice in eight year (in summer and then in winter), though apparently from different sites.
When the attack is from the same site (which it always is for kobolds and tower in my case), then the season tick is the same so far, but the normal tick can be different. Season tick is 10 normal ticks, i.e. 12 minutes in dwarf time, so the difference is very small, if it is late afternoon when the zombies come, it is always the late afternoon, even if the minute is different.
I'm not sure how time is calculated for purpose of siege, though. Distance on world map, displayed as time to reach a site, certainly uses Chebyshev's distance for calculation (i.e. traversing a diagonal takes the same time as traversing in orthogonal direction, like king's movement on a chessboard), and takes mountains and water bodies into consideration. This is different from unit movement on local map (both in adventurer and dwarf mode), where standard Euclidean metric is used (i.e. a diagonal takes about 1.41 more time to traverse than orthogonal move).
My zombies are 9 tiles away, which makes a day's travel on the map, but if they would be able to fly they would be only 5 tiles away (about half a day), or even 5.83 tiles if Euclidean metric was used. They arrive 12.6 hours after the season starts, which suggests crow's flight line (and it's hard to tell the metrics) and not the stated time-distance. However, so close site gives bigger error in calculation (
Site 28 tiles distant is slightly more than three days away (officially), so assailants should come on 4th day, but maybe the mountains are taken into consideration after all, or it uses Euclidean metrics for actual time calculation, or there's a bit of random factor involved, or the attacking site was different, or maybe something other. I don't have attackers so distant on my map, the dark fortress is three days away, and most gobbo sites are two days or less.