Insert, Update Dates in db

I have read a few of the other forums and haven't found an answer.

I am working in CS3, have super suite, php.

My question is...
Can dataAssist be used to update or calculate date fields to be inserted into the db?
Or do I need to write a seperate script to handle that.

I need to have 3 date fields in my db table.
Date created.
Subscription_Start date.
Subscription_End date.
(using DATETIME in my db table)

The subscription_end date should be able to be inserted and updated by "x" about of months purchased by the visitor/user.

How do I craft this into the dataAssist wizard...insert and update pages?
How do I get the datetime from the server?
I know that TIMESTAMP automatically inserts into the db, but haven't gotten datetime to auto insert.

Thank you in advance for any help you can give me.