Trexlim,
A true date calculation can be very involved as you have to consider leap years.
One way to solve your problem is:
1) One time only, create a "15_minutes_file".
2) Create a cron job that runs every 15 minutes.
2.a) Test if the file is older than 15_minutes_file -- "if file -ot 15_minutes_file"
2.a.I) Meets your criteria.
2.b) Update (touch) the "15_minutes_file" file with the current date.
See if this works for you.