MATLAB: Correcting timestamp from netCDF

MATLABnetcdftimeseriestimestamp

Hello to matlab the community,

If possible I would be grateful for some help in regards to conversion of timestamps in matlab from netcdf.

I am familiar with netcdf and their structure as well as their conversion, but I think I am missing something.

I have a netcdf with time in the format of

time =[23627;
23627.0069444444;
23627.0138888889;
23627.0208333333];

And I am trying to correct the timestamp both to a human readable form but also to use in plots.

Of course I checked the time offset in the netcdf and it is in

"days since 1950-01-01T00:00:00Z";

Therefore, I have included this offset in my estimates but I cannot see to be getting the correct form. I am using this

time_offset=datenum(1950,01,01);
new_time=(time/24)+time_offset; 
timestamp=datestr(new_time);
timestamp=datestr(new_time,'yyyymmddHHMM');
nt=str2num(timestamp);
timestamp=uint64(nt);
timestamp=double(nt);

However the outcome does not give me what I would expect.

The measurements should be in 10 minute increments like so:

01-01-2015 00:00:00
01-01-2015 00:10:00

The variable is new_time is estimated as

7.132084583333334e+05

that corresponds to the variable timestamp:

195209111100

It seems that it does not recognize the offset. Have I done anything wrong?

Thank you all in advance,

Best Answer

>> time = [23627 23627.0069444444 23627.0138888889 23627.0208333333];
>> dt = datetime(1950,1,1,'Format','dd-MMM-yyyy HH:mm:ss') + days(time)
dt = 
  1×4 datetime array
   09-Sep-2014 00:00:00   09-Sep-2014 00:09:59   09-Sep-2014 00:20:00   09-Sep-2014 00:29:59

I guess netCDF timestamps are UTC? The above will work for that, in general, you'd want to use caldays to account for DST shifts twice a year.

Related Solutions

MATLAB: Timezone to UTC: How to add duration vector to datetime vector

Frank, what's up is Daylight Saving Time, which occurred on March 8th this year. I'm guessing you knew that, but got mixed up in the time arithmetic. Here's what's going on. You have this:

>> time = ['2015/03/08 00:00';'2015/03/08 01:00';'2015/03/08 03:00';'2015/03/08 04:00'];
>> Time = datenum(time,'yyyy/mm/dd HH:MM')
Time =
                    736031
          736031.041666667
                736031.125
          736031.166666667

datenums don't know anything about time zones or DST. Those times mean whatever you want them to mean. Since you then do this ...

>> date = datetime(Time,'ConvertFrom','datenum','Timezone','America/Montreal')
date = 
   08-Mar-2015 00:00:00
   08-Mar-2015 01:00:00
   08-Mar-2015 03:00:00
   08-Mar-2015 04:00:00

... I assume you mean that those datenums are to be interpreted as times in the America/Montreal time zone. But what you've left out is this:

>> date.Format = [date.Format ' z']
date = 
   08-Mar-2015 00:00:00 UTC-5
   08-Mar-2015 01:00:00 UTC-5
   08-Mar-2015 03:00:00 UTC-4
   08-Mar-2015 04:00:00 UTC-4

The second and third times are only an hour apart. Then this ...

>> [UTC_Offset,dst] = tzoffset(date);
>> Date = date - UTC_Offset
Date = 
   08-Mar-2015 06:00:00 UTC-4
   08-Mar-2015 07:00:00 UTC-4
   08-Mar-2015 07:00:00 UTC-4
   08-Mar-2015 08:00:00 UTC-4

... adds 5 hours to the first and second times, but adds only 4 hours to the third and fourth times. But you're doing the addition in the America/Montreal time zone. Adding 5 hours to the first two puts them after the DST shift @2am, which makes them look like you added 6 hours. In other words, your array is not UTC, it's shifted America/Montreal. The easiest thing to do to get UTC times is this:

>> date.TimeZone = 'UTC'
date = 
   08-Mar-2015 05:00:00 UTC
   08-Mar-2015 06:00:00 UTC
   08-Mar-2015 07:00:00 UTC
   08-Mar-2015 08:00:00 UTC

Hope this helps.

MATLAB: Load time from netcdf file and retain the ‘Hour’ value

Instead of

 time = double(netcdf_time/24 + datenum('1900-01-01 00:00:00'));

(netcdf/24 is still happening in int32, hence you are loosing precision)

Try

 time = double(netcdf_time)/24 + datenum('1900-01-01 00:00:00');

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