timecolumn() function parses a time string from a column according to the set timefmt settings. The result is a time stamp in seconds.
set xdata time set timefmt "%H:%S" set format x "%H" set style fill solid 0.6 border -1 set boxwidth 0.3 relative set xrange["00:00":"23:30"] set style data boxes plot 'dataset.txt' using 1:2, \ '' using (timecolumn(1)+60*20):($2*0.5), \ '' using (timecolumn(1)+60*40):($2*0.7)