Is there a way to omit parts of a defined command?

Is there a way I can set a defined command to be able to respond to omitted parts of it without throwing an “Illegal Function” error?

You're talking about optional arguments, like many of the builtins have? You can't do that with user DEFs, unfortunately. One thing you can do everywhere is ignore required return values of an OUT function by just leaving the variable name blank, but you still need to use the commas.

'I only care about seconds
TMREAD OUT ,,SEC