I performed a generic test by taking a test document in my repository and assigning it two different date fields (original date and new date). I manually set the original date to be 9/2/2014 and left the new date blank. Then I created a test workflow that gets the original date value, then runs that token through the date token calculator to add 91 days to it and then I assigned the resulting token from the date token calculator activity into the new date field and it properly set it to be 12/2/2014.

With that being said, perhaps the issue may lie in your loop iteration and perhaps the date you're adding 91 days to isn't what you think it is. For debugging purposes, can you track the tokens in each loop iteration or perhaps record the original date value being processed along with the new calculated date and store those values in separate debug tokens and assign that someplace so you can confirm that the loop activity is being done on the proper values?

If everything looks correct and during each loop, the original date being calculated against is correct, but it's just not correctly adding 91 days to it, then please open a support case with your reseller and provide a copy of the workflow along with a sample subset of the database where you're getting the original date from. Feel free to trim things out so that nothing sensitive is passed along.