SUBTRACT_YEARS takes a date and a number of years to add, and returns a date with the given number of years added.
In the special case that the new year doesn't contain the original date (e.g. when subtracting a year from the leap day 29.02.2016) the new date will have the same number of days from the beginning of the year (e.g. 01.03.2015 in the earlier example).
This expression returns the current date 18 years ago.