[gschizas]

A simple (and stupid) /time.aspx in VB.NET, because that's what was easily available:

    <%
    Dim epochMilliseconds As Long = CLng((DateTime.UtcNow - New DateTime(1970, 1, 1)).TotalMilliseconds)
    Response.Write("{ ""epoch_ms"": " & epochMilliseconds & " }")
    %>

(you need to change "/time" to "/time.aspx" for the original HTML page to work)

[boulevard]

This is perfect, for completeness, you can extend it with CORS + no-store and ms epoch.