{Edit} removed old reference link
Seconds since Jan 1, 1970 = unix time
To get this in VB.NET see below example (in example using DateTime.UtcNow, but you can plug in any DateTime you want there)
Dim uTime As Double
uTime = (DateTime.UtcNow - New DateTime(1970, 1, 1, 0, 0, 0)).TotalSeconds
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