Cepstral Community

Is there a document that says which SSML features Cepstral supports? (besides the examples listed here: http://www.cepstral.com/en/tutorials/view/ssml)

In particular, I'm having a hard time trying to get duration to work, and I'm trying to figure out if its my mistake or the system.

The prosody tag for "duration" is not supported. You may wish to try using the "rate" tag to alter the rate of speech.
For example:
The price of the meal was <prosody rate="-20%">$45</prosody>

http://www.cepstral.com/en/tutorials/view/ssml

This contains more information on SSML. If you have further questions about any specific SSML usage, feel free to post them here, and we'll do our best to help.

Thanks!