How to convert a datenum to a datetime?

Question

JFz 2017-11-17

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https://ww2.mathworks.cn/matlabcentral/answers/367648-how-to-convert-a-datenum-to-a-datetime

评论： JFz 2017-11-20

Hi, I have table with column of datenum. I need to do a join with another table which has also a column of date but in just a plain format such as '11/1/2017'. But I got this error: Left and right key variables 'Date' and 'Date' are not comparable because one is a non-cell. How to resolve this error? Thanks a lot!

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Answer 1

Star Strider 2017-11-17

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https://ww2.mathworks.cn/matlabcentral/answers/367648-how-to-convert-a-datenum-to-a-datetime#answer_291735

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Use the 'ConvertFrom' option:

t = datetime(X,'ConvertFrom','datenum')

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JFz 2017-11-17

Works. Thanks!

Star Strider 2017-11-17

My pleasure!

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Answer 2

Peter Perkins 2017-11-20

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https://ww2.mathworks.cn/matlabcentral/answers/367648-how-to-convert-a-datenum-to-a-datetime#answer_292127

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Star Strider's answer seems like the long way around. JFz, it always helps to provide a short concrete example so people don't have to guess, but it sounds like you have one table with a datenum variable and another with a datestr variable,

>> t1 = table(floor(now) + [1;2;3],[1;2;3],'VariableNames',{'Date' 'X'})
t1 =
  3×2 table
     Date     X
    ______    _
    737020    1
    737021    2
    737022    3
>> t2 = table(datestr(floor(now) + [3;2;1]),[33;22;11],'VariableNames',{'Date' 'Y'})
t2 =
  3×2 table
       Date        Y 
    ___________    __
    [1x11 char]    33
    [1x11 char]    22
    [1x11 char]    11

and you want to join the two tables using dates as the key. Convert the dates to datetimes, and you're done:

>> t1.Date = datetime(t1.Date,'ConvertFrom','datenum')
t1 =
  3×2 table
            Date            X
    ____________________    _
    21-Nov-2017 00:00:00    1
    22-Nov-2017 00:00:00    2
    23-Nov-2017 00:00:00    3
>> t2.Date = datetime(t2.Date)
t2 =
  3×2 table
       Date        Y 
    ___________    __
    23-Nov-2017    33
    22-Nov-2017    22
    21-Nov-2017    11
>> t3 = join(t1,t2)
t3 =
  3×3 table
            Date            X    Y 
    ____________________    _    __
    21-Nov-2017 00:00:00    1    11
    22-Nov-2017 00:00:00    2    22
    23-Nov-2017 00:00:00    3    33