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POConverting between NTP and C# DateTime
 

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  1. POConverting between NTP and C# DateTime
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    <p>I use the following code to convert between NTP and a C# DateTime. I think the forward corversion is correct, but backwards is wrong.</p> <p>See the following code to convert 8 bytes into a DatTime:</p> <h2>Convert NTP to DateTime</h2> <pre><code>public static ulong GetMilliSeconds(byte[] ntpTime) { ulong intpart = 0, fractpart = 0; for (var i = 0; i &lt;= 3; i++) intpart = 256 * intpart + ntpTime[i]; for (var i = 4; i &lt;= 7; i++) fractpart = 256 * fractpart + ntpTime[i]; var milliseconds = intpart * 1000 + ((fractpart * 1000) / 0x100000000L); Debug.WriteLine("intpart: " + intpart); Debug.WriteLine("fractpart: " + fractpart); Debug.WriteLine("milliseconds: " + milliseconds); return milliseconds; } public static DateTime ConvertToDateTime(byte[] ntpTime) { var span = TimeSpan.FromMilliseconds(GetMilliSeconds(ntpTime)); var time = new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc); time += span; return time; } </code></pre> <h2>Convert from DateTime to NTP</h2> <pre><code>public static byte[] ConvertToNtp(ulong milliseconds) { ulong intpart = 0, fractpart = 0; var ntpData = new byte[8]; intpart = milliseconds / 1000; fractpart = ((milliseconds % 1000) * 0x100000000L) / 1000; Debug.WriteLine("intpart: " + intpart); Debug.WriteLine("fractpart: " + fractpart); Debug.WriteLine("milliseconds: " + milliseconds); var temp = intpart; for (var i = 3; i &gt;= 0; i--) { ntpData[i] = (byte)(temp % 256); temp = temp / 256; } temp = fractpart; for (var i = 7; i &gt;= 4; i--) { ntpData[i] = (byte)(temp % 256); temp = temp / 256; } return ntpData; } </code></pre> <p>The following input produces the output:</p> <pre><code>bytes = { 131, 170, 126, 128, 46, 197, 205, 234 } var ms = GetMilliSeconds(bytes ); var ntp = ConvertToNtp(ms) //GetMilliSeconds output milliseconds: 2208988800182 intpart: 2208988800 fractpart: 784715242 //ConvertToNtp output milliseconds: 2208988800182 intpart: 2208988800 fractpart: 781684047 </code></pre> <p>Notice that the conversion from milliseconds to fractional part is wrong. Why?</p> <h2>Update:</h2> <p>As Jonathan S. points out - it's loss of fraction. So instead of converting back and forth, I want to manipulate with the NTP timestamp directly. More specific, add milliseconds to it. I would assume the following function would do just that, but I'm having a hard time validating it. I am very unsure about the fraction-part.</p> <pre><code>public static void AddMilliSeconds(ref byte[] ntpTime, ulong millis) { ulong intpart = 0, fractpart = 0; for (var i = 0; i &lt; 4; i++) intpart = 256 * intpart + ntpTime[i]; for (var i = 4; i &lt;= 7; i++) fractpart = 256 * fractpart + ntpTime[i]; intpart += millis / 1000; fractpart += millis % 1000; var newIntpart = BitConverter.GetBytes(SwapEndianness(intpart)); var newFractpart = BitConverter.GetBytes(SwapEndianness(fractpart)); for (var i = 0; i &lt; 8; i++) { if (i &lt; 4) ntpTime[i] = newIntpart[i]; if (i &gt;= 4) ntpTime[i] = newFractpart[i - 4]; } } </code></pre>
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    1. COI've updated my answer below. Good luck!
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