I think you will automatically jump about 25-30 days before starting your next patrol (don't remember the exact number), plus a day for each percent of hull damage. When you first get back to base it shows the date that you returned, but by the time you go out again it will advance. You don't manually need to edit anything for that to happen.
SH3 Commander simply randomises the number of days, or gives you the option to choose the number of days. I'm not sure how long Type II boats stayed in port between patrols, but since they were not at sea as long I'd expect it was probably a little less than for the Type VIIs.
If you don't want to use Commander, but you do want to edit the number of days, you'll have to wait for somebody else to help because I don't know how to do that.