Keep `Months to simulate` filled if interrupted

[thexa4]

I frequently try to skip entire years but am often interrupted by events.
It would be nice if the `Months to simulate` field would prefill with the amount of months to the original destination.

For example, when skipping 12 months and getting interrupted after 3, prefill it with 9 months.

[Eric.B]

Sounds like a good cheap bounty. I'll add it to the list when I resume working on this stuff.