These timestamps are captured using the kernel scheduler clock, which counts in nanoseconds since boot. The exact details depend on the compile-time parameters chosen to build a particular Linux distribution and the target architecture.
In general, the timestamp of a sample is captured around the same time when it's recorded. Timestamps on the same core are guaranteed to be monotonically increasing as long the core remains in an active state. The samples you've shown were all captured on the same core and the core remained active from the first sample to the last sample. So the timestamps are guaranteed to be monotonic in this case irrespective of the platform and distribution. When profiling on multiple cores, there is no guarantee that the clocks on all cores are in sync.
All perf tools use the same clock to capture timestamps, but they may differ in the way timestamps are printed and it may happen that two tools print timestamps from the same sample file differently. This depends on the kernel version.
It's possible to specify a clock source when calling perf_event_open() by setting use_clockid to 1 and setting clockid to one of the clock sources defined in linux/time.h, such as CLOCK_MONOTONIC. perf record provides the -k or --clockid option to specify the clock source for capturing timestamps.
Modern distributions on x86 typically use TSC as the source for the scheduler clock (check /sys/devices/system/clocksource/clocksource0/current_clocksource). So if you're on an x86 processor, most probably the TSC of the profiled core was used to capture the current value of TSC cycles, which internally gets converted into nanoseconds. When a timestamp is printed, it may get converted to a different unit. In this case, timestamps are printed in the format "seconds.microseconds". A summary of the behavior of TSC on Intel processors can be found at: Can constant non-invariant tsc change frequency across cpu states?.
