Question about ttml times

Hi Everyone,

I am working on code to translate between TTML and WebVTT.  I have a TTML sample file that expresses the times of samples in the hours:minutes:seconds:frames format.  But, the file has no indication of what the frame rate is.  So, what is the best practice for converting times from this format into other formats, in the absence of a frame rate?  

Thanks for any help,
Courtney