On time / In time / Just in time
UsageOn time concerns the schedule; in time means early enough for a purpose; just in time means only narrowly early enough.
On time
Use when
Something happens at the scheduled or expected time.
Example
“The meeting started on time.”
In time
Use when
Something happens early enough for a particular purpose or before it is too late.
Example
“The papers arrived in time for the committee meeting.”
Just in time
Use when
Something happens only narrowly early enough, shortly before it would have been too late.
Example
“The approval arrived just in time to prevent the tender from lapsing.”
Quick check
just on time to prevent, on time when early enough for a purpose is meant, in time when exact punctuality is meant
Usage note
Use “just in time” for narrowly avoiding lateness. Use “right on time” when you mean exactly at the scheduled time; do not treat every modifier before “on time” as incorrect.