"relative" means it's relative to filtered entries only so that the
sum of shown entries will be always 100%.  "absolute" means it retains
the original value before and after the filter is applied.

For example:

Select the second 10% time slice to diff:

perf diff --time 10%/2

Select from 0% to 10% time slice to diff:

perf diff --time 0%-10%

Select the first and the second 10% time slices to diff:

perf diff --time 10%/1,10%/2

Select from 0% to 10% and 30% to 40% slices to diff:

perf diff --time 0%-10%,30%-40%

It also supports analyzing samples within a given time window
<start>,<stop>. Times have the format seconds.nanoseconds. If 'start'
is not given (i.e. time string is ',x.y') then analysis starts at
the beginning of the file. If stop time is not given (i.e. time
string is 'x.y,') then analysis goes to the end of the file.
Multiple ranges can be separated by spaces, which requires the argument
to be quoted e.g. --time "1234.567,1234.789 1235,"
Time string is'a1.b1,c1.d1:a2.b2,c2.d2'. Use ':' to separate timestamps
for different perf.data files.

For example, we get the timestamp information from 'perf script'.

perf script -i perf.data.old


  mgen 13940 [000]  3946.361400: ...

perf script -i perf.data


  mgen 13940 [000]  3971.150589 ...

perf diff --time 3946.361400,:3971.150589,

It analyzes the perf.data.old from the timestamp 3946.361400 to
the end of perf.data.old and analyzes the perf.data from the
timestamp 3971.150589 to the end of perf.data.

baseline/A compute/B compute/C  samples
---------------------------------------
b                    x          f1
b          x         x          f2
b                               f3
b          x                    f4
b                               f6


           x         x          f5

baseline/B compute/A compute/C  samples
---------------------------------------
b          x         x          f2
b          x                    f4
b                    x          f5


           x         x          f1


           x                    f3


           x                    f6

baseline/C compute/B compute/A  samples
---------------------------------------
b                    x          f1
b          x         x          f2
b          x                    f5


                     x          f3


           x         x          f4


                     x          f6