Look to the tutorial by Paul Edstein (macropod) that was linked above.
Microsoft Word Date Calculation Tutorial
It has ready-to go field coding, but read the introductory material.

See also the cross-posted question.
Redirecting

Additional resource: Dealing with Fields in Microsoft Word

You are not asking about basic stuff. It is time to dig in and learn some. That is what the tutorial is for.

• Fields in Word are dynamic bits in a document that can change based on other information without macros. In this case, a complex field changes its results based on a date input elsewhere and marked as a bookmark.
• Fields have code that tells Word which field you are using and what you want it to do. The code and the switches may be specific to the field. There are online references for each field. List of field codes in Word
• The textual representation is, I suspect, someone's typing what a field code looks like here - in text. Field codes are not text, exactly, in that the { } braces must be inserted by Word, not typed.
• Field results are what you see on the screen when a field is updated and not displayed as code.

[Legacy] Formfields and Content Controls are two ways of getting information from a user. They should not both be used in the same document/template.

• Making Forms with Microsoft Word