1
general::
Phrase(s): *on the dot
Fig. at exactly the right time. (*Typically: be somewhere ~; arrive (somewhere) ~; get somewhere ~; see someone ~; show up ~.) • I’ll be there at noon on the dot. • I expect to see you here at eight o’clock on the dot.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Exactly on time, as in We had to be there at eight on the dot. The dot in this idiom is the mark appearing on the face of a watch or clock indicating the time in question. It may come from the earlier to a dot, meaning "exactly" since the early 1700s but no longer heard today. [c. 1900] Also see ON THE BUTTON.
American Heritage Idioms