[c#] How can I convert a DateTime to the number of seconds since 1970?

I'm trying to convert a C# DateTime variable to Unix time, ie, the number of seconds since Jan 1st, 1970. It looks like a DateTime is actually implemented as the number of 'ticks' since Jan 1st, 0001.

My current thought is to subtract Jan 1st, 1970 from my DateTime like this:

TimeSpan span= DateTime.Now.Subtract(new DateTime(1970,1,1,0,0,0));
return span.TotalSeconds;

Is there a better way?

If the rest of your system is OK with DateTimeOffset instead of DateTime, there's a really convenient feature:

long unixSeconds = DateTimeOffset.Now.ToUnixTimeSeconds();

The only thing I see is that it's supposed to be since Midnight Jan 1, 1970 UTC

TimeSpan span= DateTime.Now.Subtract(new DateTime(1970,1,1,0,0,0, DateTimeKind.Utc));
return span.TotalSeconds;

That approach will be good if the date-time in question is in UTC, or represents local time in an area that has never observed daylight saving time. The DateTime difference routines do not take into account Daylight Saving Time, and consequently will regard midnight June 1 as being a multiple of 24 hours after midnight January 1. I'm unaware of anything in Windows that reports historical daylight-saving rules for the current locale, so I don't think there's any good way to correctly handle any time prior to the most recent daylight-saving rule change.

I use year 2000 instead of Epoch Time in my calculus. Working with smaller numbers is easy to store and transport and is JSON friendly.

Year 2000 was at second 946684800 of epoch time.

Year 2000 was at second 63082281600 from 1-st of Jan 0001.

DateTime.UtcNow Ticks starts from 1-st of Jan 0001

Seconds from year 2000:

DateTime.UtcNow.Ticks/10000000-63082281600

Seconds from Unix Time:

DateTime.UtcNow.Ticks/10000000-946684800

For example year 2020 is:

var year2020 = (new DateTime()).AddYears(2019).Ticks; // Because DateTime starts already at year 1

637134336000000000 Ticks since 1-st of Jan 0001

63713433600 Seconds since 1-st of Jan 0001

1577836800 Seconds since Epoch Time

631152000 Seconds since year 2000

References:

Epoch Time converter: https://www.epochconverter.com

Year 1 converter: https://www.epochconverter.com/seconds-days-since-y0

You can create a startTime and endTime of DateTime, then do endTime.Subtract(startTime). Then output your span.Seconds.

I think that should work.

You probably want to use DateTime.UtcNow to avoid timezone issue

TimeSpan span= DateTime.UtcNow.Subtract(new DateTime(1970,1,1,0,0,0));