Returns the date as a unix timestamp.
Returns Unknown if one of the parameter values is unknown or outside the common valid date boundaries.
ToUnixTimestamp(date)
ToUnixTimestamp(Now())
=>Result: 1648645519
ToUnixTimestamp(Date(2022, 05, 30))
=>Result: 1648637600
ToUnixTimestamp(unknowntimestamp)
=>Result: unknown
