help with If/Then/Else or Case select, I think

New at this and not sure if I am in the right place.

I am trying to calculate a date in the future based on an entered date and the sample falling into one or more categories.

Fields: Start Date, manually entered
Interval, combo box, choice of monthly or weekly
Pull Interval, combo box, choice of Long term, Short Term, or Stressed
Conditions, combo box, choice of 25°C, 40°C, or 60°C
1 Week, calculated field
3 Month, calculated field

What I would like to happen is that when the Interval box is the choice of monthly that the 3 Month field calculates the date 3 months from the start date. Also, if the Interval field is weekly the 3 Month field is to be left blank.
I have managed the Dateadd function to calculate the 3 month date, but do not know how to incorporate the condition of the weekly statement to leave the 3 month field blank.
Also where should I put this, right now I have this calculation in a subform. Would it be best to generate this in a query? and if so how do I set this up?

Also, I need to select that the Pull Interval is Long term that I can only choose the Conditions field, 25°C, Interval field as Monthly, and only calculate the 3 month time point. Same thing with the Short term and stressed Pull Intervals, I want when they are selected to only allow me to look at fields relative to those conditions.

Any help would be extremely appreciated. I have several books and they are not spelling out exactly what I need, and with my limited programming experience I am stuck.

Thanks
ChrisB37

There are date functions like DateAdd and DateDiff that may give you a starting point.