I know the smoke hot key is Ctrl+Alt+T to switch between timecode and frame numbers on the timeline, but I can’t for the life of me remember where the setting lives in the timeline view.
I can’t remember there being a toggle for it directly in the timeline tab but if you go into preferences > user interface there’s a “time display mode” setting that lets you choose frame or timecode.
Granted my memory may not be what it used to be, but I’ve never known there to be a tc/frame toggle on the UI for the timeline. Only by Hot key or preference.