Here is the search, putting results in a summary index.
sourcetype="SmtpPrevent_operational" dtime=*s | convert auto(dtime) | search dtime>=60 | sitimechart span=1h count(dtime>=60), count(dtime>=120), count(dtime>=180)
(longhand version)
sourcetype="SmtpPrevent_operational" dtime=*s | convert auto(dtime) | search dtime>=60 | sitimechart span=1h count(dtime>=60), count(dtime>=120), count(dtime>=180) | summaryindex spool=t uselb=t addtime=t index="dtime_plus_60" file="Summary Index DTime 60+,120+,180+ per Hour_1480261911.stash_new" name="Summary Index DTime 60+,120+,180+ per Hour" marker=""
Now here is what ends up being in the summary index:
10/18/2012 10:00:00, search_name="Summary Index DTime 60+,120+,180+ per Hour", search_now=1350576300.000, info_min_time=1350568800.000, info_max_time=1350572400.000, info_search_time=1350576324.840, psrsvd_gc=27, psrsvd_v=1
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