I need a Word macro which will look through the current document and find/replace all instances of certain words as follows:

"+today" is replaced by today's date
"+1 week" is replaced with the date one week hence
"+2 weeks" is replaced with the date two weeks hence

I don't want them updated each time the document is opened.

I have spent ages looking online for a solution but everyone says that any macro involving calculating dates is very difficult. I don't understand why this should be the case. I want to be able to copy and paste the macro so I can use it.