Exercise on standard distribution

Suppose I want to calculate the probability for a user to pass from a basic to a premium account within the end of Q1 (i.e. end of march).
I know that this is distributed with mean=50 days and st.dev=10 days, then the probability that a user pass to a premium account within the end of march (that means in 33days or less) can be calculated with excel as NORM.DIST (33,50,10, TRUE) and is 0.044. Please tell me if this is incorrect.

My question is:
How can I add another variable to my model, that is the number of days since the basic account was registered? I’d like to have two different prob for:

- User (A): days since registration= 0
- User (B): days since registration = 7

What is the value I should use as ‘x’?