Mollie, you can certainly calculate a date based on the Project Start date easy enough. It is a bit more tricky to roll-up a date within the schedule but can be done and I do it quite often. Try this:
1. Create a task custom flag field called something like “Key Project Task”
2. Set this task to Yes for the key task within your schedule. This will mark the key task
3. Create another task custom date field called “Key Project Date. Use a formula that states If “Key Project Task” = yes, Finish, else null. Roll this up to summary tasks as Maximum. This will set the task 0 task to the finish date associated with the key task.
4. One more step, create a Project custom date field that equals Key Project Date. This copies the results of the task field to the project field.
I know it sounds like a lot but it does work as long as you are limiting the number of fields you want rolled up. There are probably some blogs that explain this (and probably better) but I haven’t done a search.
Thanks for the help on this, I have finally tried to implemented it and I get the concept but I can not get the formula for step 3 correct. Or maybe it’s a problem with step 4. If I open my plan and look at the task field for the “Key Project Date” the date is transferred from the Finish date to the “Key Project Date” field for that task but is not rolled up. So when I do the Project custom date field equal to the “key Project Date” I don’t get a result. Does the Project custom date need to be equal to the max of the task field or something? I have it just equal to it.
Mollie, hard to tell what the issue is but setting it to max might help. If now, then feel free to send me a schedule with your task formulas and I can compare it to one I know works. I’m happy to help if I can. Thanks…