Skip some data

Dear all,
sorry about my question I think it is very simple, but I can not resolve my problem.

I have some histograms in my .root file. The data in this file are measurements of energy deposition of neutron in the proportional counter.
Axis X is channel number (energy)
Axis Y is number of counts.
These data were obtained for 60 minutes. But I need data only for the last 20 minutes of the measurement. Is it possible to construct a histogram skipping first values obtained at the time?

With a tree (or ntuple) you can do it. With an histogram (like TH1) you cannot.