It would be nice being able to do time based math in ME.
I would suggest writing a new macro that you can call in your current one (to keep it cleaner).
Go to text->date/time and prompt user for date. Make sure you know what format you're using.
Use variable->modify string (copy part of text into variable value 3 characters 2 would turn the selected variable into 15 for 08/15/2012)
Do this for all 3 dates and you'll have numbers you can start working with and modifying.
in the new macro for the date math you'd have set up 3 variables, use an if statement to check what month it is and if the day goes over 30 for the appropriate months, it adds 1 to the month code also. Then set each of the new numbers into a set of variables to call in your main script on writing the new date.
The best solution would be to use from the current day. Then you can just use text-> date then set one variable for number of days into the future, etc. But this only works from the current day, if that can be changed combining date/time's promp and date's days into the future setting then it would be better, but I don't know how that can be done.
If you're willing to change todays date on your system to the first day you want to use, it'll work better, but that's also more work.
I think it also depends on how you want it displayed. June 16, 2012 or 06/16/2012 etc.