How can I obtain the current TAI time in milliseconds in Linux using either Java or C++?
The reason I need this is to be able to accurately take timestamps over a long period of time (on the order of years) and still be able to compare them, without worrying about leap seconds. It is possible for multiple measurements to take place during a leap second and all measurements need to be unambiguous, monotonically increasing, and linearly increasing. This will be a dedicated Linux server. This is for a scientific project which needs precision of about .5 seconds.
I do not currently wish to invest in a GPS timekeeper and hope to use NTP to pool.ntp.org in order to keep the system clock on track.
I have looked into the following solutions:
Java 8 or the ThreeTen Project The only way to obtain a TAIInstant is to use an Instant and then convert it which, according to the specs, "Conversion from an Instant will not be completely accurate near a leap second in accordance with UTC-SLS." That in and of itself is not a big deal (in fact, using UTC-SLS would also be acceptable). However, using now() in the Instant class also seems to just be a wrapper for System.currentTimeMillis(), which makes me think that during the leap second, the time will still be ambiguous and the project will not actually give me TAI time. The Java 8 specifications also state:
Implementations of the Java time-scale using the JSR-310 API are not required to provide any clock that is sub-second accurate, or that progresses monotonically or smoothly. Implementations are therefore not required to actually perform the UTC-SLS slew or to otherwise be aware of leap seconds.
Using a right/? timezone This seems like it would work, however I am not sure if the implementation is smart enough to continue working during a leap second or if System.currentTimeMillis() would even give TAI time. In other words, would the underlying implementation still use UTC, thus giving an ambiguous time during the leap second which is then converted to TAI, or does using a right/ timezone actually work with TAI using System.currentTimeMillis() always (ie even during leap second)?
Using CLOCK_TAI I tried using CLOCK_TAI in the Linux kernel but found it to be completely identical to CLOCK_REALTIME in my test: Code:
#include <iostream>
#include <time.h>
long sec(int clock)
{
struct timespec gettime_now;
clock_gettime(clock, &gettime_now);
return gettime_now.tv_sec;
}
int main()
{
std::cout << sec(0) << std::endl; // CLOCK_REALTIME
std::cout << sec(1) << std::endl; // CLOCK_MONOTONIC
std::cout << sec(11) << std::endl; // CLOCK_TAI
return 0;
}
The output was simply:
1427744797
6896
1427744797
Using CLOCK_MONOTONIC The problem with this is that the timestamps need to remain valid and comparable even if the computer restarts.