In addition to being able to specify 'day' as the newTimeStep input to retime you can specify a newTimes vector. Similarly, in groupsummary you can specify the groupbins input as 'day' or as a list of bin edges. Let's make a newTimes vector / vector of bin edges.
Start off with a vector of random datetime values.
d = datetime('now') + hours(48*randn(10, 1));
Let's get the earliest and latest datetime values in d.
firstTime = min(d);
lastTime = max(d);
Shift the earliest datetime to the start of its day (midnight) then add six hours. Similarly, shift the latest datetime to the end of its day and add six hours.
firstSixAM = dateshift(firstTime, 'start', 'day') + hours(6);
lastSixAM = dateshift(lastTime, 'end', 'day') + hours(6);
Actually, we need to shift firstSixAM earlier if the earliest time occurs before 6 AM on its day. In that case, dateshift determined midnight on that day but then the added six hours stepped past the earliest time. So we actually need 6 AM the day before.
if firstTime < firstSixAM
firstSixAM = firstSixAM - days(1);
end
The allSixAMs vector created below contains a vector of datetime values, spaced one day apart, each of which represents 6 AM on that date. You can use this as the newTimes for a retime call or as the bin edges for groupsummary.
allSixAMs = firstSixAM:days(1):lastSixAM;
We can check that every element of d is in the range spanned by the elements of allSixAMs.
whichBin = discretize(d, allSixAMs);
t = timetable(d, whichBin, allSixAMs(whichBin).', allSixAMs(whichBin+1).', ...
'VariableNames', {'BinNumber', 'StartOfBin', 'EndOfBin'})
The value of d in each row in t should be between the value of StartOfBin and EndOfBin in that row.
