DateTime's representation in milliseconds?

I have a SQL-server timestamp that I need to convert into a representation of time in milliseconds since 1970. Can I do this with plain SQL? If not, I've extracted it into a DateTime variable in C#. Is it possible to get a millisec representation of this ?

Thanks,
Teja.

You're probably trying to convert to a UNIX-like timestamp, which are in UTC:

yourDateTime.ToUniversalTime().Subtract(
    new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)
    ).TotalMilliseconds

This also avoids summertime issues, since UTC doesn't have those.

In C#, you can write

(long)(date - new DateTime(1970, 1, 1)).TotalMilliseconds

As of .NET 4.6, you can use a DateTimeOffset object to get the unix milliseconds. It has a constructor which takes a DateTime object, so you can just pass in your object as demonstrated below.

DateTime yourDateTime;
long yourDateTimeMilliseconds = new DateTimeOffset(yourDateTime).ToUnixTimeMilliseconds();

As noted in other answers, make sure yourDateTime has the correct Kind specified, or use .ToUniversalTime() to convert it to UTC time first.

[Here][1] you can learn more about DateTimeOffset.

[1]: https://msdn.microsoft.com/en-us/library/system.datetimeoffset(v=vs.110).aspx "MSDN DateTimeOffset Struct"

SELECT CAST(DATEDIFF(S, '1970-01-01', SYSDATETIME()) AS BIGINT) * 1000

This does not give you full precision, but DATEDIFF(MS... causes overflow. If seconds are good enough, this should do it.

This other solution for covert datetime to unixtimestampmillis C#.

private static readonly DateTime UnixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

public static long GetCurrentUnixTimestampMillis()
{
    DateTime localDateTime, univDateTime;
    localDateTime = DateTime.Now;          
    univDateTime = localDateTime.ToUniversalTime();
    return (long)(univDateTime - UnixEpoch).TotalMilliseconds;
} 

There are ToUnixTime() and ToUnixTimeMs() methods in DateTimeExtensions class

DateTime.UtcNow.ToUnixTimeMs()

Using the answer of Andoma, this is what I'm doing

You can create a Struct or a Class like this one

struct Date
    {
        public static double GetTime(DateTime dateTime)
        {
            return dateTime.ToUniversalTime().Subtract(new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)).TotalMilliseconds;
        }

        public static DateTime DateTimeParse(double milliseconds)
        {
            return new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc).AddMilliseconds(milliseconds).ToLocalTime();
        }

    }

And you can use this in your code as following

DateTime dateTime = DateTime.Now;

double total = Date.GetTime(dateTime);

dateTime = Date.DateTimeParse(total);

I hope this help you