The numeric system used in Bangladesh is called Bengali. No formula is needed to make a date conversion: the only difference is in the way the numbers are written.
Typical date inscription on a coin of Bangladesh looks like this (the year on the picture below is 1993 A.D.):
As part of a typeface, Bengali digits are written as follows:
The system of coin denominations
The currency of Bangladesh is the taka. One taka is subdivided into 100 paisa or poisha. Both takas and poishas are minted, though poishas are rarely used — because of their small value and therefore low purchasing power.