Magick Source

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h1. Convert NTP 64 bits time to UnixTime

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I was working on a "RTP":http://www.ietf.org/rfc/rfc3550.txt related project, recieving Reports, and RTP uses NTP 64 bits timestamps. But those are not very useful for me, so I needed to convert them to UnixTime Stamps.

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After googling a lot for a way to transform one Stamp to the other, ended up reading the source for the "OpenMash":http://www.openmash.org/, more specificly, the "rtp/ntp-time.h":http://www.openmash.org/lxr/source/rtp/ntp-time.h header file.

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So, this post try to explain what each of this formats are, and how to convert from the first to the later.

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h2. NTP 64 bits

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The 64 bits NTP TimeStamp is a two double-words value. The high 32 bits represent the seconds from January 1st, 1900 and the second represent the fractions of seconds. NTP 64 bits is a high precision value, capable of represent fraction with 1/4294967296 of second precision.

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h2. Unix TimeStamp

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The Unix TimeStamp represent the number of seconds from January 1st, 1970, and are not usually used to represent smaller than a second time fractions. However, in Perl, you can use the fractional part, but it will be ignored, I think.

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h2. Converting NTP 64 Bits to Unix TimeStamp

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<pre><code class="perl">

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my $ntphi=3439021312;

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my $ntplo=3302649461;

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my $ntpts=$ntphi+$ntplo/4294967296;

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my $uxts=$ntpts-2208988800;

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my $localdate=localtime($uxts);

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</code></pre>

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h2. Explaining the code

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First, we create the real representation of the NTP TimeStamp, with a integer part of seconds and a decimal part representing parts of seconds.

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Then remove the offset (in seconds) between January 1st, 1900 and January 1st, 1970 (2208988800).

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This give us the unix timestamp.

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Obviously, if you just want the unixtimestamp on a second precision, all you need is the high part of the ntp timestamp (the hi 32 bits part), and to substract the offset.

NTP64bits2UnixTime