Fit Contributor question

Hi all,

I'm trying to setData in a Fitcontributor field with a time format (more or less).

I work with elapsedTime and select a part of it in order to expose minutes and seconds of some specific session part in the watch. This milliseconds I convert as string (36000ms as 0:36s, ie) and I'm trying to save this data as string in a FitContributor.DATA_TYPE_STRING, but I've got an error "setData string too long for allocated space".
Somebody know how can I save this value in a fit field?
Maybe I must to save in milliseconds and convert somehow?

Any light to move foward will be very appreciate!