rechtzeitig
in time
Early enough or at the right moment, so that something is still possible or no disadvantage arises.
Wir müssen rechtzeitig losfahren, um den Zug nicht zu verpassen.
We have to leave in time so we don't miss the train.

in time / on time
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in time
Early enough or at the right moment, so that something is still possible or no disadvantage arises.
Wir müssen rechtzeitig losfahren, um den Zug nicht zu verpassen.
We have to leave in time so we don't miss the train.

The word is made up of 'recht' and 'zeitig'. Here, 'recht' means 'right' or 'appropriate'. So 'rechtzeitig' (in time) literally means 'at the right time'.
Danke für die rechtzeitige Warnung!
Thank you for the timely warning!
As an adverb, 'rechtzeitig' (in time) can stand in different positions in the sentence. It often stands directly before the verb or action it describes, or in position 1 for emphasis.
Wir haben das Problem noch rechtzeitig erkannt.
We recognized the problem just in time.
'Rechtzeitig' (in time) is often combined with verbs that involve an appointment or a deadline. Typical phrases are: 'rechtzeitig ankommen' (to arrive in time), 'rechtzeitig fertig werden' (to finish in time), or 'rechtzeitig Bescheid sagen' (to let someone know in time).
Bitte sag mir rechtzeitig Bescheid, ob du kommst.
Please let me know in time whether you're coming.
late
'spät' and 'rechtzeitig' both have to do with an important point in time. 'spät' means that something happens after the expected or agreed time. 'rechtzeitig' means that something happens early enough so that it still works out or no problem arises. The words can be confused because both occur with appointments, departures, or deadlines. The difference is: 'rechtzeitig' looks at the goal and says that there is still enough time left; 'spät' says that the appropriate time has already passed.

Die Kundin ruft rechtzeitig an, damit wir den Termin noch verschieben können.
The customer calls in time so that we can still reschedule the appointment.
Im Urlaub merkt sie zu spät, dass ihr Pass abgelaufen ist.
On vacation, she realizes too late that her passport has expired.