I think I am correct in assuming that the "star" you are referring to looks like this:
Since in the Stock game, which GFO basically is, with some important fixes applied, only has the one mission area between New Ireland and New Britain. When looking at that particular mission in the Mission Editor, it looks like this:
(well, after merging the Campaign_NMS and Campaign_LOC it does - anyway...)
The assignment you got for this came from the DynamicMiss.cfg file, and that has the same mission effective dates as the PatrolObjectives.cfg file, which are from 19420801 to 19440301, so you fall within those parameters. The actual "time" for that patrol is for 2880 minutes continuous, which equates to 48 hours, or 2 days. But it is "continuous", which means that if you leave the "zone" (that "circle" above), the time then starts over. The Objective and all parameters appear to be valid, and I did run a test mission with it, and it does complete. However, notice that the patrol zone (circle) is 100 kilometers in radius, not 100 nautical miles. The 100 kilometers in radius equates to just under 54 nautical miles. More like 53.99+nm, which is rather small in radius, hence the two day time frame (most likely).
What some folks do when doing some missions, is they will go to the star, and "camp-out" there for the duration, just to make certain their orders fulfill, then go do what they want. Another thing to remember about GFO (Stock), is that you only earn 200 renown points for this particular mission Objective, so sinking a large merchant, or a light cruiser (enemy, of course) will earn you that with one or two torps. You ain't missing much dropping it and heading for home. Next time you fone home and get an assignment, it will usually be documented on the radio, so you should be able to go back through the radio log and at least see some of the constraints imposed by your orders. The stock game does often fail to tell you how big of a Patrol Zone you are dealing with though, but suffice to say that if you draw a 50nm radius circle, which would be 100nm in diameter, you should be OK.
