Setting two clocks to the same time doesn’t seem that hard, right? If same-second precision is enough for you then you can usually do that. But when we are talking nanosecond precision like that provided by PTP (IEEE1588, Precision Time Protocol) there are some more variables to consider.


The first variable to note is the time it takes to set the new time. Even if the action to set the time is triggered at the exact same time, the OS will introduce latency in the operation. Anything that depends on the OS being precise in its operations is out of the question. For this reason the adjustments should be done through adjustments rather than setting an absolute time. By knowing the difference between two clocks the time can be adjusted by that much to synchronize them. Doing this with PTP is explained in this post.

Assuming the initial difference calculated is precise then this will bring the time difference hopefully within maybe a microsecond.


Now that the clocks are nearly aligned we need to consider the frequency of the oscillators driving the clocks. If two clocks are driven by oscillators at different frequencies, or even just of different qualities, it will result in wander. A big difference will allow significant wander in between every synchronization, resulting in never reaching near-perfect accuracy. When the clocks are within a certain threshold time adjustment can stop and clock syntonization starts. The slave clocks will adjust their operating frequency. Though not literally the frequency, but rather a compensation value the clock will take into account when advancing its time. The frequency is adjusted so the time difference between the clocks approaches 0. If the slave time is behind it needs to increase the frequency gradually until it catches up, at which point it will start fluctuating back and fourth just around the frequency of the master.


Clock holdover time is how long it can stay within a certain offset from the master after losing the synchronization (e.g. losing connection). A typical requirement is that it should keep some accuracy during 5 seconds loss. In terms of PTP this may happen if its master clock goes down and it needs to find an new master. And during those 5 seconds while waiting for a new master it cannot drift too far away from the initial time.

Physical Hardware Clock

When requiring nanosecond-precision timekeeping a hardware clock is used. In Linux this is referred to as a Physical Hardware Clock (PHC). It keeps the time and allows some operations to manage it.

  1. Set time
  2. Get time
  3. Adjust time
  4. Set frequency

Points 1) and 2) are not required by PTP, but are useful for debugging. 3) is the one that is used to adjust for the difference. 4) sets the frequency of the PHC. These operations can be accessed using the phc_ctl tool from the Linuxptp project (along with some more operations).

In Linux the PHC are exposed as devices under /dev, typically as /dev/ptpN, where N denotes the number of the PHC.

Set time

The time set is an absolute value in Unix time. Meaning a value of 10 sets the time to 1970-01-01 00:00:10.

phc_ctl /dev/ptp0 set 10

Get time

Reads out the time

phc_ctl /dev/ptp0 get

Adjust time

Increments or decrements the clock by the given time in seconds, read as double precision floating point values.

phc_ctl /dev/ptp0 adj 10.5

Set frequency

Sets the frequency of the clock in ppb (parts-per-billion), meaning how many nanoseconds per second it should it should stray from its real frequency. Note that each PHC has different maximum frequency adjustments. The PHC capabilities can be viewed with phc_ctl /dev/ptp0.

phc_ctl /dev/ptp0 freq 1222

Time scales and leap seconds

International Atomic Time (TAI) operates based on a weighted average of over 450 atomic clocks spread out around earth. Universal Coordinated Time (UTC) is based on the actual earths rotation. In TAI a day is defined as having exactly 86 400 seconds, while UTC is subject to leap seconds. Leap seconds occur due to earths rotation speed always changing slightly due to geological or climatic changes, e.g. the continental plates moving or the ice caps melting. Since this cannot be predicted over longer periods a leap second is only announced 6 months in advance.

The leap seconds always happen on either June 30th or December 31st at 23:59:59. A leap second ahead is shown as 23:59:60, before advancing to 00:00:00. A leap second backwards goes directly from 23:59:58 to 00:00:00. As for when a leap second occurs, a Linux system should be informed by an NTP daemon when one is scheduled, which in turn gets its time through NTP from a more authoritative clock device.

PTP operates on the TAI timescale, but always informs of the current UTC offset (37 at the time of writing) to make sure any devices that need the time in UTC can convert to it.