Java System.currentTimeMillis() equivalent in C#

An alternative:

// Use DateTime.UnixEpoch as of .NET or .NET Core 2.1 upwards;
// not available in .NET Framework
private static readonly DateTime Jan1st1970 = new DateTime
    (1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

public static long CurrentTimeMillis()
{
    return (long) (DateTime.UtcNow - Jan1st1970).TotalMilliseconds;
}