In Rulebook (and any LSL script, actually),
- the wallclock time is just the time in SLT
- the time of day refers to the SL day, which is 4 hours long (so the clock runs 6 times faster than a real-world clock).
So you'd use "wall clock" if you want to trigger an action at a specific SLT, whereas if you want to trigger an action depending on if it is dark or bright in SL, you'd use the wallclock time.