You can do it using DATEDIFF
function like below :
select DATEDIFF(s, '1970-01-01 00:00:00', '2017-11-20 23:12:02.000') as EpochTimeStamp
Output :
EpochTimeStamp
--------------
1511219522
You know already how can we get back the original date :
SELECT DATEADD(ss, 1511219522, '19700101') as OriginalDate
OriginalDate
-----------------------
2017-11-20 23:12:02.000
solved How do I convert a SQL server timestamp to an epoch timestamp? [duplicate]