Mapping conversion help needed for date

Hi Experts,

I need help in message mapping convesion for date field

The source date field can have value '2010-06-04T02:09:59.610-07:00' or 2010-06-04T02:09:60.610-07:00

I have to change it to '2010-06-04T02:09:59.000-07:00'. ie, if the seconds are 59.XXX, or 60.XXX, then change to 59.000.

Please let me know the best way to do this

Thanks

Mike