>>It is Numeric & Continuous type
If your date column is numeric/continuous as you have specified, then it is already in seconds, behind the scenes as it were. You must have specified a column format of m/d/y h:m:s. If you want to display seconds, then use a numerical format.
Here's a table where the first column uses the date format, and the second one uses numeric format:
New Table( "Test Date", Add Rows( 2 ),
New Column( "Date Format", Numeric, "Continuous",
Format( "m/d/y h:m:s", 23, 0 ),
Input Format( "m/d/y h:m:s", 0 ),
Set Values( [3771562821, 3771562821] )
),
New Column( "Date in Seconds", Numeric, "Continuous", Format( "Best", 12 ),
Formula( :Date Format )
)
);