I just finished an 18 hour march. I set timer to alert me when it was time to take a look at the game. I came in and the timer was off by about 4 minutes. No worries…
So I sat around on the client, waited for the 4 minutes .The time expired, and I wasn’t able to capture the settlement.
Ok, no worries, I’ll refresh the client. So I refreshed the web page. And there was no timer. So I set the unit to “Move” to the settlement it is on top of. A new timer appeared that said 8 minutes 
Okay, I’ll wait another 8 minutes. So I wait it out. The timer runs out… still can’t capture the city. Okay, I’ll refresh the client yet again.
This time it did the trick! And I was able to capture the settlement! Huzzah! 
The ETA Timer for units arriving seems to be a little off calculation wise.