MATLAB: How to convert time column from hours to decimal days to HH:MM:SS

converting time

I need to convert the first column in the excel sheet to HH:MM:SS, this is so I can plot a time series for surface water (Column B) and bottom water (Column C) temperatures.
There's two excel sheets for the years 2014 and 2015.
3 columns of data: time (days since January 1st), surface temperature, near-bed temperature
I'm not sure where to start!

Best Answer

Try this for the 2014 file (do essentially the same for any others), noting that with this format for ‘filename’, the code will automatically extract the years: 
filename = 'ccs_temperatures2014.xlsx';
T1 = readtable(filename, 'VariableNamingRule','preserve');
fileyear = regexp(filename, '\d*', 'match');
fyear = str2double(fileyear{:});
dn = datenum([repmat([fyear 0 0], size(T1,1), 1) T1{:,1} zeros(size(T1,1),2)]);
Col1 = datetime(dn, 'ConvertFrom','datenum', 'Format','HH:mm:ss');
T2 = table(Col1);
T2 = [T2,T1(:,2:end)];
T2.Properties.VariableNames = {'Time (HH:mm:ss:)','Surface Temperature (°C)', 'Bottom Temperature (°C)'};
that with: 
FirstFiveRows = T2(1:5,:)
produces: 
FirstFiveRows =
  5×3 table
    Time (HH:mm:ss:)    Surface Temperature (°C)    Bottom Temperature (°C)
    ________________    ________________________    _______________________
        13:00:00                 9.5343                     9.4499         
        13:00:12                 9.5223                     9.4494         
        13:00:24                 9.5344                     9.4493         
        13:00:37                 9.5315                     9.4489         
        13:00:50                 9.5251                      9.448
The dates and times were (again) the problem, however with a bit of creativity, that was relatively easily dealt with.
Here, only the 'HH:mm:ss' displays, although the full date information is retained internally.
EDIT — (6 Dec020 at 21:05)
I misread it the first time.
Change to: 
dn = datenum([repmat([fyear 0], size(T1,1), 1) T1{:,1} zeros(size(T1,1),3)]);
and then: 
FirstFiveRows = T2(1:5,:)
produces: 
FirstFiveRows =
  5×3 table
    Time (HH:mm:ss:)    Surface Temperature (°C)    Bottom Temperature (°C)
    ________________    ________________________    _______________________
        00:00:00                 9.5343                     9.4499         
        00:05:02                 9.5223                     9.4494         
        00:09:56                 9.5344                     9.4493         
        00:14:58                 9.5315                     9.4489         
        00:20:00                 9.5251                      9.448
.
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