Excluding date from date and time

Hello,

I am fascing difficulties with plotting data. My data are containg measurements of heart rate. First column is string with date and time, and second column are values of heart rate. I have converted first column into datetime values using following code:

day89 = datenum(heartrate_89.start_time, 'yyyy-mm-dd HH:MM:SS.FFF');

ts2 = day89;

ts2 = datetime(ts2,'TimeZone','local',...

'ConvertFrom','datenum');

day89=ts2

I have measurements for couple of days and since all of them are staring from 20:00 and last till 10:00, I was wondering if there is a way to plot a graph starting from 20:00 untill 10:00, containing traces for couple of days, because when I try to plot different traces on same graph, since the dates are different, they are far away from each other?

Is there a way to completely exclude date value since I am only interested in time?

Thank you

[Luis J Gilarranz]

can you post a excerpt of the data?

[Tea]

Yes, for example, data are starting from 8th of May (20:00) and finishing on 9th of May (around 10:00), which is one night.

I have couple of nights starting from same time and ending at same time.

I hope this picture will help.

Answer 1

[Rik]

You can probably also do this with the datetime class, but when you still have your time in the datenum format, you can simply subtract the floor to keep only time.

If you want to keep the traces continuous you can add 1 to values befor 10 AM.

day89 = datenum(heartrate_89.start_time, 'yyyy-mm-dd HH:MM:SS.FFF');

%crop to 1 day
day89 = day89-floor(day89);
L= day89 <= 10/24;
day89(L)=day89(L)+1;

ts2 = day89;
ts2 = datetime(ts2,'TimeZone','local',...
    'ConvertFrom','datenum');
day89=ts2

[Tea]

Thank you, this solved my problem.

[Laukik Avinash Kharche]

Hii All,

I need help in similar kind of situation. My experiment was done from 16:00 to 15:00 uhr next day. The time is extracted by me from excel to matlab and then I use the datetime function to convert to datetime. The time is correctly converted but the date introduced is 31st Dec 2000. I am not sure why is that happening. Even if I try to ignore the date and use the HH:mm:ss.SSS format for the datetime variable, the date 31st Dec is plotted on the Graph which I dont want. Can somebody help ?

Thank you,

Laukik.

[Peter Perkins]

You should post this as a new question in a new thread, and you need to provide more details.

Answer 2

[the cyclist]

You should be able to use the timeofday function.

The resulting output will be a duration array.

[Peter Perkins]

the cyclist is correct. While Rik's trick works, sort of, it also creates datetimes that are in the (ISO) year -1.

>> dn = now
dn =
   7.3758e+05
>> dn0 = dn - floor(dn)
dn0 =
      0.39464
>> dt = datetime(dn0,'ConvertFrom','datenum')
dt = 
  datetime
   31-Dec--0001 09:28:17

That will eventually come back to bit you. durations are the way to go.