You can write for example date()+1 will return the date of tomorow, the date
field + or - will add or substruct the date by days.
But why not use the function dateadd
e.g
=dateadd("d",2,date()) - will return the date + 2 days
=dateadd("d",-2,date()) will return the date - 2 days
=dateadd("m",2, date()) will return the date + 2 month
etc, check help on the subject, its better then using + and -