get millisecond timestamp

[Vxider]

Thank you for your Tutorial. This code will result ntp time in seconds. How can I get millisecond timestamp, I don't know how to combine txTm_s and txTm_f. And if NTP server's network latency is less then 1ms, the ntp time error in this code will less then 1ms ?

[lettier]

Hello @Vxider. I am glad you enjoyed the tutorial.

So as you know the txTm_s value is the number of seconds since the NTP epoch (1900-01-01 00:00:00) and we subtract 70 years worth of seconds to get the number of seconds since the UNIX Epoch (1970-01-01 00:00:00).

The txTm_f value represents a fraction or proportion of a second. It is unit less in terms of the standard sub-second units of time. According to the RFC, one second == 2^32-1 == 4294967295 == 11111111111111111111111111111111 (32 bits) [0]. Depending on your needs, you could go with timeval which uses microseconds or timespec which uses nanoseconds [1].

Still, if you want to combine the two and convert the unit of measure to milliseconds you could do the following.

  // Seconds since UNIX epoch.
  uint32_t txTm = packet.txTm_s - NTP_TIMESTAMP_DELTA;
  // Convert seconds to milliseconds.
  double milliseconds = (double) txTm * 1000.0;
  // Add fractional part.
  milliseconds += ((double) packet.txTm_f / 4294967295) * 1000.0;
  printf("Milliseconds since UNIX epoch: %f\n", milliseconds);

You can check the result with this tool.

For dealing with latency, see issue #4.

According to the RFC, "With NTPv4, servers and clients are precise within a few tens of milliseconds with poll intervals up to 36 hours." [0]

👍

[0] https://tools.ietf.org/search/rfc5905
[1] https://www.gnu.org/software/libc/manual/html_node/Elapsed-Time.html

[Vxider]

Thanks for your explanation, and it solved my problem. 👍