Converting a time series TGraph plot into TH1F hist

ROOT
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10

You can add custom time labels using SetBinLabel, see ROOT: TH1 Class Reference

Something like if ix15x60%500 == 0 h->SetBinLabel(i, t.Convert());

Alternatively, maybe this is easier, you can activate the time format:
see Time axis for histograms - #5 by couet