Early enough that a mistranslated program name can be corrected without pressure. Transcripts covering several years of study take longer than a single diploma, and reissuing a corrected employer letter takes the employer's own time on top of that.
Build in a round of correction
Plan on two passes rather than one: the translation arrives, it is read against the listing and the employer letter, and something is adjusted. Institutions may also need time to issue certified copies of the originals. Setting the sequence backwards from the intended start date, with room for one correction cycle, is more realistic than assuming the first version will be final.
Work backwards from the intended start and mark the dates that belong to someone else. A registrar controls when a certified copy is issued; the employer controls when a corrected letter is signed; only the translation itself sits with the person managing the file. Hypothetical example: a scientific journal's production schedule fixes a start in the first week of a quarter, and the transcript request alone consumes several weeks because the studies span three programmes.
Give the correction cycle its own slot in that plan instead of borrowing time from the end. If a date slips, tell the employer before the letter is reissued, since a letter written to the old chronology creates a second inconsistency to explain.