If you're not bothered about dates before 1970, or millisecond precision, just do:
-- SQL Server
SELECT DATEDIFF(s, '1970-01-01 00:00:00', DateField)
Almost as simple as MySQL's built-in function:
-- MySQL
SELECT UNIX_TIMESTAMP(DateField);
Other languages (Oracle, PostgreSQL, etc): How to get the current epoch time in ...
If you need millisecond precision (SQL Server 2016/13.x and later):
SELECT DATEDIFF_BIG(ms, '1970-01-01 00:00:00', DateField)
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