• CAST(DATEDIFF(s, '1970-01-01 00:00:00.000', @InputDate) AS BIGINT) * 1000 + DATEPART(ms, @InputDate) AS 'MS Since 1970',
• convert(bigint,datediff(S,convert(datetime,'01/01/1970 00:00:000'),A.DateTimeModified)) *1000 AS 'MS Since 1970' 

http://www.databasejournal.com/features/mssql/article.php/3820771/Formatted-date-in-SQL-Server-2008.htm

DATEDIFF (datepart ,startdate ,enddate )

datepart

Is the part of startdate and enddate that specifies the type of boundary crossed. The following table lists all valid datepart arguments. User-defined variable equivalents are not valid.

startdate

Is an expression that can be resolved to a time, date, smalldatetime, datetime, datetime2, or datetimeoffset value. date can be an expression, column expression, user-defined variable or string literal. startdate is subtracted from enddate.

To avoid ambiguity, use four-digit years. For information about two digits years, see two digit year cutoff Option.

enddate

See startdate.