The date() function returns the current system date in internal format as an integer number representing the number of days that have elapsed since December 31, 1967 (day 0 on the D3 calendar).
date()
The parentheses following the function are required, and must not contain arguments. Without the parentheses, date is assumed to be a variable.
This calculates the number of days elapsed between the value of invoice.date and the current system date.
number.days = date() - invoice.date
This calculates the number of weeks elapsed between the value of invoice.date and the current system date.
weeks.old = (date() - invoice.date) / 7