Good example. Thank you, @Tids.
For anyone else reading, there are other things to consider too. For example, if you send your units to a trip, but then the global speedup or your own prestige expire before they hit, their return trip will be slower and thus take more time.
Even an edge case such as your Bastion getting downgraded while your troops are travelling can have a (minor) impact. There are many moving parts in this game and it's hard to consider all that are relevant.