Ruby on Rails, How to reset a counter every day?

I need to save a reference number every time I save a record of a certain model, the reference should be composed of 10 numbers, the first 8 numbers are related to the creator id and date, but the last 2 digits should be an incremental number starting at 00 and ending at 99, this count should be reset every single day.

For example:

Records created the same day:
SD01011800
GF01011801
MT01011802
...
GH01011899
------------------------------------------------------------------------------
Records created the next day:
SD02011800
GF02011801
MT02011802
...
GH02011899

Where the first 2 letters are the initials of a name, the next 2 are the current day, next 2 current month, next 2 current year, next 2 incremental number (from 0 to 99, reset daily)

Also every reference HAS TO be unique. I'm missing the last two digits part, any idea on how to grant this ? Thanks for reading.