[Arcorann]

MAX_SAFE_INTEGER = 2^53 - 1 = 9007199254740991. The next integer 2^53 is representable, while the following one 2^53 + 1 isn't.

2^53 + 1 milliseconds = 9007199254740993 milliseconds = 104249991 days 8 hours 59 minutes 0.993 seconds. Since the zero point is midnight 1970-01-01, we reach the first non-representable millisecond at 287396-10-12T8:59:00.993 (be careful to use the Gregorian leap rule if you want to calculate this manually).