A man used to live with a calendar and 24-hour representation of time. Coordinated Universal Time (UTC) is used for that now. These details are not needed for most kernel or application processes, so there is multiple time sources available for tracing tools:

Time source DTrace SystemTap
System timer is responsible for handling periodical events in kernel such as context switch. System timer usually ticks at constant frequency (but ticks may be omitted in tickless kernels). Interval between firing timer is usually referred as special unit of time: tick, lbolt in Solaris or jiffy in Linux. Timer frequency in Linux can be get using HZ() function. `lbolt or `lbolt64 jiffies()
Processor cycles counter is a special CPU register which act as a counter which increases on each cycle, such as TSC in x86 or %tick in SPARC. It may not be monotonic. get_cycles()
Monotonic time. Starts at unspecified moment of time (usually at system boot), but ticks with constant intervals. May use high-resolution time source such as HPET on x86, but may impose some jitter between CPU cores or CPUs. timestamp local_clock_<unit>() or cpu_clock_<unit>(<cpu>)
Virtual monotonic time of thread. Similar to previous time source, but only accounts when thread is on CPU, which is useful to calculate CPU usage of a thread vtimestamp
Real time or Wall-clock time. Monotonic time source which starting point is an UNIX Epoch (00:00:00 UTC, Thursday, 1 January 1970). May use extra locks, access RTC, so it generally slower than previous time sources walltimestamp gettimeofday_<unit>()

In this examples <unit> is one of (s –- seconds, ms –- milliseconds, us –- microseconds and ns –- nanoseconds). DTrace time sources always have nanosecond resolution.

Generally speaking, monotonic time sources are better for measurement relative time intervals, while real time is used if you need precise timestamp of an event (i.e. for cross-referencing with logs). To print a real timestamp, use ctime() function in SystemTap which converts time to string, or use %Y format specifier in DTrace print functions.