Anyway to add the day ordinal suffix to the time format?

[Ciced]

Hello all. I'm trying to add the ordinal suffix to the day of any timestamp, for example currently my timestamp reads:

Posted: 12 December 2005

But I want it to read:

Posted: 12th December 2005

With the ordinal suffix changing for the required day (1st, 2nd, 3rd, etc). SMF 1.0.5 seems to be using a modified php date() function so I'm having a spot of bother trying to find out where I can add any additional %variables. Can anybody solve my problem? Any help appreciated.