Dear all, please help me with a formula

i need to calculate the leave balance for all my team members, however, the
leave keeps adding up on a monthly basis. so i need a formula, which finds
the difference between today(present day) and the date of joining to
calculate the leave.

i can use a particular date and find the difference, however, if i have
calculated it for today, next month, i will hve to rework it again for that
month's date.

this seems a lil complicated, can anyone help me out please