Add Days Formula Not Working

I saw this page in a similar question, but when I follow those instructions, I get a syntax error. Here is the reference page with the instructions I followed:

https://sfmanual.rednao.com/doc/conditional-logic/#529

I’m wanting to take a date and add 210 days (or 7 months). I tried both 210 days and 7 months, and I get the same error message each time.

1. I click the little square icon to the right of the Default Value field. This opens up the formula box.

2. I then select the name of the date field I want to add 210 days to.

3. I type in a period (.) and select either the AddDays or AddMonths selection. I put that number inside the parenthesis, so either one looks like this:

[field_rnField6].AddMonths(7)
[field_rnField6].AddDays(210)

4. When using either one of the formulas above, I click “Validate,” but I get this message:

“An error occurred. Syntax error: Invalid or unexpected token.”