Units are your choice
Unix tools and most server languages count seconds; JavaScript and Java count milliseconds; databases and tracing systems often use micro- or nanoseconds. The same digits mean dates thousands of years apart depending on the unit, so this converter never switches units by itself. It warns when a value is implausible for the unit you picked.
Values are parsed as big integers, so nanosecond digits are shown exactly instead of being rounded through a floating-point number.
Limits
Dates are supported in the ECMAScript range of ±8,640,000,000,000,000 ms (about ±273,790 years from 1970), using the proleptic Gregorian calendar. Negative values are before 1970. Leap seconds are not counted, as on POSIX systems; a time like 23:59:60 is not representable.