Convert Unix Timestamp to .net int64 datetime

First of all, I know that it's duplicate, but those questions aren't what I am looking for.

I'm using an API to add participants in an online application. When I post JSON object to endpoint, everything is posted except dates. There's some misunderstanding with date format. If I get existing participants and check the date format that's this:

{
   "participant":{  
      "ID":201954,
         //  Long Object
      "DateOfBirth":"\/Date(857305424000-0500)\/",
   }
}

I figured out that first 9 char are UNIX timestamp but rest of them?? not sure. Any idea what is this format?

EDIT Online application is made in .net/asp/c# not sure about them. But I am writing my code in php application which post json object via cURL

EDIT 2 I checked parsing wrong formats and got this error:

The value '0.48631700 1425319593' cannot be parsed as the type 'Int64'.'. Please see InnerException for more details.

Any idea how can I get the format of .net's int64 datetime in php?

Solution

JSON itself does not specify how dates should be formatted, so you need to know the requirements of the target .NET application. Most likely it makes sense to convert to a "readable" date format

Example (php):

$timestamp=1333699439;
echo gmdate("Y-m-d\TH:i:s\Z", $timestamp); 
// outputs 2012-04-06T08:03:59Z

Example (.net):

DateTime d;
DateTime.TryParse("2012-04-06T08:03:59Z", out d); 
// receives 4/6/2012 8:03:59 AM (in US-format)

Edit:

.NET int64 format is a number of ticks since midnight 0001-01-01 00:00:00 (every tick is 1/10000000 of second) and UNIX timestamp is number of seconds since beginning of the UNIX epoch (1970-01-01 01:00:00), so desired result is $seconds = 621355968000000000/10000000 - $number_of_seconds, see Convert ticks to unix timestamp and https://kramerc.com/2010/11/30/converting-datetime-ticks-into-a-unix-timestamp-in-php/

Therefore, for JSON output you should do

$timestamp=1333699439;
$ticks = ($timestamp * 10000000) + 621355968000000000;
// outputs 634692962390000000

Test function php

function ticks_to_time($ticks) {
    return floor(($ticks - 621355968000000000) / 10000000);
}

$time = ticks_to_time(634692962390000000);
echo date("F j Y g:i:s A T", $time);
// outputs April 6 2012 4:03:59 AM EDT

Test function .net

Int64 intDate = 634692962390000000; 
DateTime dt = new DateTime(intDate); 
Console.WriteLine(dt.ToString()); 
// outputs 4/6/2012 8:03:59 AM