If you see the time remaining to a certain event is 00:00:0?, there is an eventjam.
Why does it occur? Edit
Eventjams are due to server overload.
If a lot of events are happening on the server, it can not immediately calculate the time remaining to an event, such as the return of your troops.
There's nothing you can do to avoid it.
How is it resolved? Edit
The calculations are queued and will be carried out in the correct order by the server. This means that if your troops were due to arrive in 2 minutes, they will still arrive before someone else's troops that were due to arrive in 3 minutes, even though neither of you will see this occur at the exact right second.
How long is the event queue? Edit
To find out how many events are currently in the queue, visit this link (for com7). If you click the 00:00:0? itself, that page of the manual will be opened saying something like "At the moment 394 events await to get calculated."
If you hover your mouse over the figure, you will see 4 other numbers that total the event queue number. For example, 109|66|6|213. If you know what these mean, please edit this page.