T-SQL DATEDIFF_BIG - 469ms diff with currentmills and documented start-end constraints incorrect?
The documentation for datediff_big is wrong. The limits specified is for datediff.

The millisecond value of 593 that you see is an artefact of the site CurrentMills that get the datetime value of now with millisecond precision and the keeps that value.

Use this link https://currentmillis.com/?now multiple times and navigate to 1970-01-01 and see that you get different rest values each time.