ClickUp reads and provides dates in various formats during imports and exports. This doc explains the three formats you'll see.
Normal
Day, date, time, timezone offset
English formatting
Due dates without specific times are technically due at 4am in the task creator's local timezone
Example: May 30, 2019 PST would be Monday, May 30th 2019, 4:00:00 am -07:00
ISO (International Standard)
Year, month, day, (T) time
Date numbers separated by a hyphen ("-")
Time numbers separated by a colon (":")
Date numbers less than 10 are preceded with a zero
Example: May 30, 2019 would be 2019-05-30T11:00:00.000Z
POSIX
The number of milliseconds that have elapsed since January 1, 1970
This format is the easiest for machines to read and perform calculations and comparisons with
Also known as Unix time, Unix epoch, or Unix timestamp
Amazing converting tool: https://www.epochconverter.com/
Example: May 30, 2019 would be 1559178061000