A couple of preliminaries:
xlsread is not recommended. readtimetable is what you want.
'200007130000' isn't a "datenum", that term means something very specific in MATLAB. Also 'yymmddHHMMSS' can't be write unless you are using zero-based month numbers, which ... don't. To convert that string to a datetime just do this (I'm just guessing on the format):
>> datetime('200007130000',"InputFormat","yyyyMMddHHmm","Format","yyyy-MMM-dd HH:mm")
So given that, you may not actually be able to use readtimetable, you may need to use readtable to read those timestamps in as text, and then convert them as above, and then use table2timetable.
OK, so you have a timetable, named tt. I don't really know specifically what you mean by "I need the time durations that the component is in operation, above a certain height", but you can use old-school logical operations to find runs of heights > threshold, and then use timetable and table operations to get their durations.
First set up some fake data.
>> WaveHeight = sin(0:.5:50)';
>> tt = timetable(WaveHeight,'TimeStep',minutes(1));
Now find the rows of that timetable with tall waves, then find the start and end of the runs of tall waves, then remove all the other rows.
>> overThreshold = tt.WaveHeight > H;
>> tt.RunStarts = [false; diff(overThreshold)>0];
>> tt.RunEnds = [diff(overThreshold)<0;false];
>> tt.RunNumber = cumsum(tt.RunStarts) .* overThreshold;
>> runs = tt(tt.RunStarts | tt.RunEnds,["RunStarts" "RunEnds" "RunNumber"])
Time RunStarts RunEnds RunNumber
______ _________ _______ _________
Now set things up conveniently, and se unstack to reorganize the data to have each row represent one run, and get the durations.
>> runs.RunStartEnd = categorical(1*runs.RunStarts + 2*runs.RunEnds,[1 2],["Start" "End"]);
Time RunNumber RunStartEnd
______ _________ ___________
>> runs = timetable2table(runs);
>> runs = unstack(runs,"Time","RunStartEnd");
>> runs.Duration = runs.End - runs.Start
RunNumber Start End Duration
_________ ______ ______ ________
I'm just guessing at what you have and what you want, but this should get you started.