I get date info when plot a Datetime in x-axis.

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Victor Olivero
Victor Olivero on 2 Apr 2021
Commented: dpb on 5 Apr 2021
Hi, I am trying to plot a voltage data vs time data (HH:MN:SS) from a Datetime, but when I build the graph the x-axis always take the Year data (Dec 31, 1899).
I have tried modified the Datetime and the axis parameters, but always get this info.
How can I only display Hour:Minutes:Seconds? The year info is not important for me.
Thanks.
  4 Comments
dpb
dpb on 5 Apr 2021
Yes, that too. A MATLAB datetime doesn't exist without an associated date and time.
Unlike the venerable and deprecated datenum or an Excel date that are both represented as a julian date in a double where the whole portion of the value represents days and the non-integer part the fractional portion of a 24-hour day, the datetime is an object containing both a date and a time and the constructor for it will create a date from thin air if one isn't provided.
timeofday() does the equivalent conversion to a duration object as the alternative suggested -- the difference is the subtraction of the origin of the timeseries I suggested will give you a 0 origin irregardless of the actual time, timeofday will return the actual hours of the day of the datetime values.
Which is to use if not just the fixup of the display of the datae depends on situation.

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Accepted Answer

dpb
dpb on 2 Apr 2021
Edited: dpb on 2 Apr 2021
Another case of TMW being too aggressive in hiding things from the end user inside graphics objects -- the year/date 'SecondaryLabel' property is hidden and is shown by default for all datetime axes.
Use
hAx=gca; % retrieve the current axes handle
hAx.XAxis.SecondaryLabel.Visible='off';
to make the year marker not be visible.
The alternative is to subtract the first time value of your datetime vector from all elements and convert to a duration and plot against it instead of the absolute time values.
dt=Time2-Time2(1); % convert to elapsed time; is a ML duration, not a datetime
plot(dt,v_bacteria)
  1 Comment
Victor Olivero
Victor Olivero on 4 Apr 2021
Thanks for your reply, I have solved the issue taking, for example, plot(timeofday(Time2),V_bateria) using the timeofday fuction. However, I will try your recomends.

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