How do I determine the number of seconds since 1/1/1970?

This is a pretty common one, as many systems use this measure for various date calculations. Here are examples in VBScript, JScript and Transact-SQL. Note that to get the number of milliseconds since 1/1/1970, you need to multiply these results by 1000.  
 
VBScript

<%      timeStart = "1/1/1970 12:00:00 AM"      Response.Write(datediff("s", timeStart, now()))  %>

JScript - we need to correct for UTC, which is already accounted for.

<script language='jscript' runat=server>      var timeStart = new Date();      var tz = timeStart.getTimezoneOffset()*60*1000;      timeStart = Math.floor(((timeStart.getTime() - tz))/1000);      Response.Write(timeStart);  </script>

Transact-SQL

SELECT DATEDIFF(SECOND, '1/1/1970 12:00:00 AM', GETDATE())

Note also that these solutions do not account for leap seconds. You can add them in manually for leap seconds that have been known to occur in the past (06/30/1997 and 12/31/1998 are two examples). You can not predict them programmatically... since leap seconds are defined by the variability of the Earth's rotation, and are declared by the International Earth Rotation Service as required. See this Google posting and this U.S. Navy article for highly detailed information about leap seconds and how they affect UTC. After reading these, you may see why your calculations don't need to be this precise -- especially since your web and/or SQL Server are probably not set to atomic clocks anyway. :-)