Add a variable amount of time to a date field via rules
The interval module provides an action "Apply an interval to a date" (you find it in the "data"-section of the action list). The module supplies a "interval field", but surprisingly, instead of providing integration between rules and that field, it provides that action, which provides integration between rules and an integer field. The action allows you to specify a data value and specify which interval that number is to be interpreted as (days, months, ...)