In case of timeseries data, axis tic position values must be given as quoted dates or times according to the format timefmt. If the <start>, <incr>, <end> form is used, <start> and <end> must be given according to timefmt, but <incr> must be in seconds. Times will be written out according to the format given on `set format`, however.
Examples:
set xdata time
set timefmt "%d/%m"
set xtics format "%b %d"
set xrange ["01/12":"06/12"]
set xtics "01/12", 172800, "05/12"
set xdata time
set timefmt "%d/%m"
set xtics format "%b %d"
set xrange ["01/12":"06/12"]
set xtics ("01/12", "" "03/12", "05/12")
Both of these will produce tics "Dec 1", "Dec 3", and "Dec 5", but in the second example the tic at "Dec 3" will be unlabelled.