In a CW carrier, the time jitter is figured out by integrating the phase noise, thereby converting it to pk-pk or rms time jitter.
If one is deliberately FM modulating a carrier, then that FM modulation will easily dominate over the phase noise effect. So all you have to do is analyze the fm modualtion and you will have the answer. You need to know the deviation and the moudlatating frequency(s). If the deviation is large, then small angle approximations do not apply and you will have a lot of time jitter!