date and time - Histogram of runs

What I would like to do is generate a histogram of the consecutive time spent under a level. For example if the level I was interested in was 1.0 and I had this data;

{{{2010, 1, 1, 6, 15, 0.}, 0.04375}, {{2010, 1, 1, 6, 30, 0.},0.04375}, 
{{2010, 1, 1, 6, 45, 0.},0.04375}, {{2010, 1, 1, 7, 0, 0.},5},
{{2010, 1, 1, 7, 15, 0.}, 0.5},{2010, 1, 1, 7, 30, 0.}, 5}}}

I would get a histogram containing two values, 45 (7:00am - 6:15am) and 15 (7:30am - 7:15am) minutes.

Any help would be appreciated.

Answer

Split time-series data based on a one-place test function, and collect total durations for spells of periods satisfying the test condition:

 ClearAll[durationsF];
 durationsF[timeseries : {{{__}, _} ..}, test_] :=
    With[{ts =  timeseries /. {{dt__}, val_} :> {AbsoluteTime[{dt}], test[val]}},
    (Last@# - First@#) & /@  Map[First, Split[ts, #[[2]] === True &], {2}]/60 /. 
      (0) ->  Sequence[]];

OP's example:

 opdata = {{{2010, 1, 1, 6, 15, 0.}, 0.04375}, {{2010, 1, 1, 6, 30, 0.},  0.04375},
   {{2010, 1, 1, 6, 45, 0.},  0.04375}, {{2010, 1, 1, 7, 0, 0.}, 5}, 

   {{2010, 1, 1, 7, 15, 0.},  0.5}, {{2010, 1, 1, 7, 30, 0.}, 5}};
 durationsF[opdata , # <= 1 &]
 (* {45, 15} *)

Weather data example:

 psdnWthr = WeatherData["Pasadena",  "Temperature", {{2012, 1, 1}, {2012, 1, 15}}];
 DateListPlot[psdnWthr, GridLines -> {Automatic, {15}}, Joined -> True]

enter image description here

 durationsF[psdnWthr, # <= 15 &]

 (* {180, 1200, 2460, 840, 480, 720, 120, 1980, 3840, 1020} *)

Notes: If the input time series is not sorted, use ts=SortBy[ts,First] in the function definition. For units of measurement other than minutes, multiply by appropriate factors.

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