:up: 'It's time...' refers to a convention/tradition/belief/common practice.... So 'It's time the kids were in bed' means they ought to be and they're not. If that convention/tradition/belief/common practice... has been badly violated, you can say 'It's high time...'. A less extreme violation - 'It's about time...'.
If they ought to be [according to that convention/tradition/belief/common practice...] but might not be (you just don't know) you can use the same form of words, but with the present tense: 'It's [the] time the night watchman [usually] has his nap, so it should be safe'.
b
PS The :up: was for B - I hadn't read the latest post when I mentioned 'about' as if it was something new.