Unable to assign number to date type field

[Gasman]

there is also VBA converter here:
Excel VBA: Convert a date string to a Unix timestamp - Stack Overflow

also here:
vba date to unix timestamp Code Example (codegrepper.com)

That appears to give the same as DateAdd and "s"

? exceldatetime(200834905 ,False)
13/05/1976 11:28:25

? dateadd("s",200834905,#01/01/1970#)
13/05/1976 11:28:25