Which box of fmp4 file contains CTTS information similar to that of ordinary MP4 file？

I converted fmp4 files into TS files by imitating the code of converting MP4 files into TS files. I encountered a problem: There is CTTS box in MP4 file, but there is no CTTS box in fmp4 file. The information inside CTTS (PTS = STTS + CTTS) is needed when calculating video PTS. What should I do? My h264 frame has no B frame. Do I need a CTTS box?

fmp4 encodes timestamps, flags and sizes in the trun box