set timefmt "<format string>" show timefmt
The string argument tells gnuplot how to read timedata from the datafile. The valid formats are:
|Time Series timedata Format Specifiers|
||day of the month, 1-31|
||month of the year, 1-12|
||day of the year, 1-365|
||seconds since the Unix epoch (1970-01-01 00:00 UTC)|
||three-character abbreviation of the name of the month|
||name of the month|
Spaces are treated slightly differently. A space in the string stands for zero or more whitespace characters in the file. That is, "%H %M" can be used to read "1220" and "12 20" as well as "12 20".
Each set of non-blank characters in the timedata counts as one column in the using n:n specification. Thus 11:11 25/12/76 21.0 consists of three columns. To avoid confusion, gnuplot requires that you provide a complete using specification if your file contains timedata.
Since gnuplot cannot read non-numerical text, if the date format includes the day or month in words, the format string must exclude this text. But it can still be printed with the "%a", "%A", "%b", or "%B" specifier: see set format (p. ) for more details about these and other options for printing timedata. (gnuplot will determine the proper month and weekday from the numerical values.)
See also set xdata (p. ) and Time/date (p. ) for more information.
set timefmt "%d/%m/%Y\t%H:%M"
http://www.gnuplot.info/demo/timedat.htmltime data demo.