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)