[osayidan]

There most certainly is.

add a new promotion >

select your user group that is going to be affected,
ignore reputation if you wish, and if so set rep level to 0

days registered set to whatever time you had in mind.

posts set to 0 if you don't intend for them to need posts as well as time.

promotion strategy should be "posts or reputation or date", so it will only look at date in this case.

the last 2 fields are self explanatory.

To test if it works, set up a sample one of 1 day. And make sure your script to update user promotions is active (i forgot where this is but it's on by default i think).