Joe asked:

>suppose my session number is 21 and the demurrage hours have decremented  to 0. Will the shipment be made?

The next day (session number) which is evenly divisible by 4.

24. 

Which with a one-day travel time would expire on 27, causing the next use to be on 28.  Then 31 and 32 ....

With a more likely two-day travel-time, travel+demurrage expire on 29 and order on 32; then travel+demurrage expire on 37 and order on 40; then expire on 45 and order on 48 .....

~~~

You could also add in the "month" <g>, making a shipment truly rare.   There is a 'minimum' usage of a car-order, so even though the combination of frequency, demurrage-free-time and month could make a car-order a once-in-10,000 year event, I have programmed the minimum usage to be "100" days (sessions).

            Jim