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What is the equivalent of NSDate's 'timeIntervalSince1970' in C#

I need to store dates in a form of number of seconds since 1970.
With this I am getting number of seconds since 1970 with Swift by using Foundation's NSDate:
NSDate().timeIntervalSince1970
And maybe a dumb question but why this is double shouldn't it be int?
What is equivalent of this method in C#?
I am not sure what to use to get the same value.

TimeSpan t = (DateTime.UtcNow – new DateTime(1970, 1, 1));
long timestamp = (long) t.TotalSeconds;
I used the UtcNow property to ensure that the timestamp is the same regardless of what timezone this code is being run in.
Also, use the largest integer type you can find since the current epoch time is slightly less than 32 bit signed integer and you want code to be future proof.
If you do have .NET 4.6 or above, try this:
DateTimeOffset.UtcNow.ToUnixTimeSeconds()

This might work.
(DateTime.Now - new DateTime(1970,1,1)).TotalSeconds
This gets the date and subtracts the default epoch time of C# (01-01-01:00:00:00) making it start from 01-01-1970.
This is most probably the easiest way to get the same value.

Julian Time stamp to Date Time

I am trying to convert to Julian Time Stamp to Date Time. I have the following microseconds time stamp 212302469304212709. As i understand i need to add these milliseconds to the beginning of Julian Calendar (January 1, 4713 B.C., 12:00 (noon)). So i have the following method:
private DateTime GetDateTime(string julianTimeStamp)
{
var julianMilliseconds = Convert.ToDouble(julianTimeStamp)/1000;
var beginningOfTimes = new DateTime(1, 1, 1, 0, 0, 0, 0);
var dateTime = beginningOfTimes.AddMilliseconds(julianMilliseconds).AddYears(-4713).AddMonths(-1).AddDays(-1).AddHours(-12);
return dateTime;
}
Assume i pass 212302469304212709 string as the parameter. The expected result should be 2015/07(July)/01 00:08:24.212. Based on my method, i have almost the same result, but day is not 1, it is 6. Same problem for different time stamps i tested.
Could any one tell me what i am doing wrong? Thanks in advance.
Edited:
This is the exact date time i expect to receive: 2015(year) 7(month) 1(day) 0(hour) 8(minute) 24(second) 212(millisecond) 709(microsecond)

The given timestamp 212,302,469,304,212,709 μs when converted to days (just divide by 86,400,000,000) gives 2457204.505836 days (to six decimal places, which is the best I can do without a lot of extra trouble). Using the Multi Year Computer Interactive Almanac (MICA) written by the United States Naval Observatory, and putting in the free form date 2015(year) 7(month) 1(day) 0(hour) 8(minute) 24(second) 212(millisecond) 709(microsecond), the program calculates exactly the same day count (to six decimal places), proving the time stamp is an accurate Julian date.
One problem with the OP's calculation is trying to use the DateTime class before the earliest supported date, as pointed out by another poster. Also, the OP didn't say if 1 July 2015 was in the Julian or Gregorian calendar, but the MICA calculation proves it is in the Gregorian calendar. Since the OP is working in the Gregorian calendar, the epoch of Julian dates should be stated in the Gregorian proleptic calendar: Noon Universal Time, November 24, 4714 BC. The oft-quoted date January 1, 4713 BC is a proleptic Julian calendar date.
"Proleptic" means a date has been found by beginning at a modern date, who's calendar date is known with absolute certainty, and applying the rules of the chosen calendar backward until the desired date is reached, even though the desired date is before the chosen calendar was invented.

DateTime uses Gregorian calendar, so when you substract years, months and so on you are doing it with that calendar, not the Julian.
Unfortunately DateTime does not support dates before year 1. You can check the library in this post, maybe it helps you.

Converting datetime to time in C# / ASP.NET

I am trying to insert time on my asp.net project.
RequestUpdateEmployeeDTR requestUpdateEmployeeDTR = new RequestUpdateEmployeeDTR();
requestUpdateEmployeeDTR.AttendanceDeducID = int.Parse(txtAttendanceDeducID.Text);
requestUpdateEmployeeDTR.TimeInChange = txtTimeOutChange.Text;
requestUpdateEmployeeDTR.TimeOutChange = txtTimeOutChange.Text;
TimeInChange and TimeOutChange are DateTime data types. But I am inserting a time data type. How can I convert that into a time data type using C#? Thanks!

The .NET Framework does not have a native Time data type to represent a time of day. You will have to decide between one of the three following options:
Option 1
Use a DateTime type, and ignore the date portion. Pick a date that's outside of a normal range of values for your application. I typically use 0001-01-01, which is conveniently available as DateTime.MinValue.
If you are parsing a time from a string, the easiest way to do this is with the DateTimeStyles.NoCurrentDateDefault option. Without this option, it would use today's date instead of the min date.
DateTime myTime = DateTime.Parse("12:34", CultureInfo.InvariantCulture,
DateTimeStyles.NoCurrentDateDefault);
// Result: 0001-01-01 12:34:00
Of course, if you prefer to use today's date, you can do that. I just think it confuses the issue because you might be looking to apply this to some other date entirely.
Note that once you have a DateTime value, you can use the .TimeOfDay property to get at just the time portion, represented as a TimeSpan, which leads to option 2...
Option 2
Use a TimeSpan type, but be careful in how you interpret it. Understand that TimeSpan is first and foremost a type for representing an elapsed duration of time, not a time of day. That means it can store more than 24 hours, and it can also store negative values to represent moving backwards in time.
When you use it as a time of day, you might be inclined to think of it as "elapsed time since midnight". This, however, will get you into trouble because there are days where midnight does not exist in the local time zone.
For example, October 20th 2013 in Brazil started at 1:00 AM due to daylight saving time. So a TimeSpan of 8:00 on this day would actually have been only 7 hours elapsed since 1:00, not 8 hours elapsed since midnight.
Even in the United States, for locations that use daylight saving time, this value is misleading. For example, November 3rd 2013 in Los Angeles had a duplicated hour for when DST rolled back. So a TimeSpan of 8:00 on this day would actually had 9 hours elapsed since midnight.
So if you use this option, just be careful to treat it as the representative time value that matches a clock, and not as "time elapsed since midnight".
You can get it directly from a string with the following code:
TimeSpan myTime = TimeSpan.Parse("12:34", CultureInfo.InvariantCulture);
Option 3
Use a library that has a true "time of day" type. You'll find this in Noda Time, which offers a much better API for working with date and time in .NET.
The type that represents a "time of day" is called LocalTime, and you can get one from a string like this:
var pattern = LocalTimePattern.CreateWithInvariantCulture("HH:mm");
LocalTime myTime = pattern.Parse("12:34").Value;
Since it appears from your question that you are working with time and attendance data, I strongly suggest you use Noda Time for all your date and time needs. It will force you to put more thought into what you are doing. In the process, you will avoid the pitfalls that can come about with the built-in date/time types.
If you are storing a Time type in your database (such as SQL server), that gets translated as a TimeSpan in .Net. So if you go with this option, you'll need to convert the LocalTime to a TimeSpan as follows:
TimeSpan ts = new TimeSpan(myTime.TickOfDay);

Convert Human readable date into an epoch time stamp

I am working on a C# project where I have a date/time in the format of 2012-11-24 15:35:18 and I need to convert this into an epoch time stamp.
Everything I've found on Google is to convert an epoch time stamp into a human readable but I need it to be done the other way round.
Thanks for any help you can provide.

I found this here:
epoch = (DateTime.Now.ToUniversalTime().Ticks - 621355968000000000) / 10000000;
Instead of DateTime.Now, you should be able to input your desired time.

You didn't say your exact use case, but the standard .NET DateTime has a Ticks attribute which is defined as:
The value of this property represents the number of 100-nanosecond
intervals that have elapsed since 12:00:00 midnight, January 1, 0001,
which represents DateTime.MinValue. It does not include the number of
ticks that are attributable to leap seconds.
This is essentially an epoch based time, if it will suit your needs. Otherwise, with this value, you should be easily able to compute a conversion to another epoch time keeping method.

You need to use TryParse:
string input = "2012-11-24 15:35:18";
DateTime dateTime;
if (DateTime.TryParse(input, out dateTime))
{
ulong epoch = (dateTime.ToUniversalTime().Ticks - 621355968000000000) / 10000000;
}

what is the standard way to display time span over 24 hours?

I am looking for a way to correctly display localized duration of time larger than 24 hours in C# and Delphi. I mean, the DateTime type often has such string function provided, but time span does not, why?
And I don't even know what is a correct format for non localized time span.
e.g. for a duration of 2 days 10 hours 30 minutes.
I found some would display such time span as 2.10:30:00 or 2:10:30:00 or 2 10:30:00.
Please help, thanks.

For invariant culture (non localized):
myDateTime.ToString(CultureInfo.InvariantCulture)
Otherwise, just use myDateTime.ToShortDateString() or .ToLongDateString().. it will use :
System.Threading.Thread.CurrentThread.CurrentUICulture in that case..
or if you had something specific in mind, you could do
var myCulture = CultureInfo.GetCultureInfo("en-GB");
dt.ToString(myCulture);
EDIT
For TimeSpans:
Example:
DateTime dt1 = new DateTime(2012, 7, 17, 12, 30, 0);
DateTime dt2 = new DateTime(2012, 7, 18, 14, 40, 0);
TimeSpan ts = dt2 - dt1;
string s = ts.ToString("G", CultureInfo.InvariantCulture);
More examples here: http://msdn.microsoft.com/en-us/library/dd784379.aspx
EDIT 2:
More info on TimeSpan format strings:
http://msdn.microsoft.com/en-us/library/ee372286.aspx

You could follow ISO 8601, a PDF of the spec can be found here. See section 4.4.3 Duration
However, using the convention as lined out in this document looks fairly complicated to me, because according to this specification, you need to include context about when the time interval takes place:
The duration of a calendar year, a calendar month, a calendar week or
a calendar day depends on its position in the calendar. Therefore,
the exact duration of a nominal duration can only be evaluated if the
duration of the calendar years, calendar months, calendar weeks or
calendar days used are known.
If the actual moment at which the time span takes place is not relevant for your application, then maybe it would be good enough to forget about the year and month parts and just stick with the format nnDTnnHnnMnnS. That should not be too hard to achieve using the standard TimeSpan methods.
Update:
Reading a bit further, this standard does not seem to be applicable to your problem after all:
Duration is used as a component in representations of time intervals
and recurring time intervals, representation of duration as such is
not facilitated by this International Standard.
I will just leave the answer here to avoid anybody else walking into this dead end like I did, or if you think 'sort of following a standard' is good enough...