Simple question for Excel users

[Mandee04]

Hello! So, I am new to excel and basically know nothing. So please respond as if you giving a child instructions.

I am trying to create a document/spreadsheet that will automatically fill in the next "due date" of an ongoing assignment. 

I would like to input the start date and have excel auto-populate 90 days out from that date, which would be the due date of the next plan.

I have been playing around and just can't seem to figure it out.

It would be SUPER helpful if someone could briefly explain, please.

Also, I am on a regular Dell laptop. 

Thank you SO MUCH in advance! 

[#2]

@Mandee04 It can be as easy as in the picture below.

The start date is in A1 and B1 contains a formula =A1+90

Beware that the date in A1 must be a real date that is recognised by Excel. Now, your laptop is probably set up to work with a date format that is commonly used in your region, US-style (m/d/y), European-style (d/m/y) or ISO (y/m/d). When you do enter a date in the "correct" format, Excel will display it as you are used to, but in the background it has converted it to a number, counting the days from January 1, 1900. Hence, April 30, 2021 is day number 44316. And because it's a number you can just add 90 to it to arrive at the due date. Excel knows you are working with dates and formats the value in B1 as a date. But again, it actually is the number 44406.

[#3]

Thank you SO much!!! I really appreciate you taking the time to help me out with this!
1. Follow-up question...
Is there a way to make the forumal repeat? Or do I have to manually enter it in every time?
For example, is there a way to put the current start dates under column A and have column B automatically add the 90 days?
OR no?

Again, thank you so much for your time and consideration in responding and helping me with this! I greatly appreciate you! @Riny_van_Eekelen

[#4]

@Mandee04 Hover the mouse pointer over the small square in the bottom right-hand corner of the cell with the formula (B1). Click and hold and drag down as far as needed. In B2, it will say =A2+90, in B3 it will be =A3+90 etc.