Getting SyntaxError when I use date features

I have a form setup with two Date fields
1. Start Date with default set as 1 (i.e. today plus 1)
2. End Date

for the End Date field value I want it to be set as Start Date plus 1
And in the formula Editor for End Date I have Start Date.AddDays(1) but when I click validate I get “SyntaxError: Invalid or unexpected token”

I have tried deleting recreating the date fields but it’s just not working, I badly need to get this resolved