Creating a time frame

Dear excel gurus/geniuses,
I'm new to excel (as in, new in discovering what a powerful software it is!!)
I want to calculate approximate time (day?)frame for appointments in excel e.g.
If a person came in on 21/01/2018, his next appointment should be within minimum of 3 weeks and maximum of 5 weeks and I would like to make a schedule for a year, so that we can make sure he follows a set schedule.
I can add 21 days to the date in, lets say A2, and tgen 35 days to the date in A3 but now I have 2 dates to create min of 3 wks n max 5 weeks from.
Is there an easy way to do this? Is there a formula that I can apply to the columns?

Desperately need input! Your help will be greatly appreciated.

Thanks,

Ann