orosee

+1 Using Kerbal Alarm Clock, I came out of time warp 18 days before my launch date.

Confirmed (also mentions this in the Reddit Wiki): 6 Kerbals and (important! This one drove me crazy many times because I keep forgetting it) the LARGE probe module. In my case, I had stations become command centers on and off because sometimes I had a ship with the large probe module docked (it worked) and later undocked (it stopped working). Anyway that means if you've forgotten to add the module to your ship/station, all you need is to dock one large probe module to a spare docking port and it should become an active command center. Also would confirm that Mission Control shows up as "unknown target" but otherwise works.