You can also use a spread sheet. For example in Microsoft Excel;
insert the date in cell A1, insert 180 in cell A2, enter the formula
"+A1+A2" in cell A3. Dates are stored as numbers. You can do
calculations on them. This even works on many cell phones. After you
have the repack date, you can create a reminder using your Email's
calendar feature. For example, if using Windows Mail, open Windows
Calendar, move to the appropriate date, double click on a time to
create a new appointment, change the details to "Repack chute" or
something similar, and set the reminder to one or two weeks. You'll
get an alert one to two weeks before the repack is due.

Charlie