Date to Millisecond Calculators
If you need to do date math across timezones, follow this link to a couple date/timezone tools at the bottom of this page.
The known data:
Convert a millisecond value to a date string
Date to milliseconds converter
What date would that be?
A temporal value converted to milliseconds
Milliseconds converted to a temporal value
How much time between now and then?
How much time between now and then (using a millisecond date value)?
Date and Time Math Across Timezones
Date and Time Plus/Minus Quantity of Time