posix/unix time to datetime

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Hi!
I just purchased a device to register some signals and I need to know the TimeStamp of the samples.
The program gives me two options to export the data (.csv and .mat).
The problem comes with the .mat format. I choose to export the data synchronized, with unix format, and I don't get the same datetime as if I export it in csv (which is the datetime that I know is correct). For convenience I need to export the data into a .mat, and also I want to know why I don't get the same datetimes. I've been looking for this and can't find the solution. BTW, I need millisecond resolution.
For example:
0) From the .csv (which I know it is correct), the datetime samples are: (var: TimestampSync_FormattedUnix_CAL)
2023/03/20 12:06:04.100
2023/03/20 12:06:04.102
2023/03/20 12:06:04.104
...
1) From the .mat I get: ( var: TimestampSync_Unix_CAL)
1679310364100.53
1679310364102.48
1679310364104.43
...
2) Which I try to convert to datetime as:
formatted_timeStamp = datetime ( TimestampSync_Unix_CAL , 'convertfrom','posixtime' , 'Format','dd-MMM-yyyy HH:mm:ss.SSS');
and the result is:
'24-Mar-55185 13:08:20.526'
'24-Mar-55185 13:08:22.480'
'24-Mar-55185 13:08:24.433'
...
I attach the variables I'm using in case it helps.
Thank you very much in advance

Accepted Answer

Stephen23
Stephen23 on 20 Mar 2023
Edited: Stephen23 on 20 Mar 2023
Unix time is actually defined as the number of seconds since the epoch. The times you show are the milliseconds since the epoch. MATLAB uses the standard UNIX/POSIX definition.
You can easily make the conversion yourself, here are two approaches:
V = [1679310364100.53;1679310364102.48;1679310364104.43];
D = datetime(V/1000, 'convertfrom','posixtime', 'Format','dd-MMM-yyyy HH:mm:ss.SSS')
D = 3×1 datetime array
20-Mar-2023 11:06:04.100 20-Mar-2023 11:06:04.102 20-Mar-2023 11:06:04.104
E = datetime(1970,1,1);
D = datetime(V, 'convertfrom','epochtime', 'Epoch',E,'TicksPerSecond',1000, 'Format','dd-MMM-yyyy HH:mm:ss.SSS')
D = 3×1 datetime array
20-Mar-2023 11:06:04.100 20-Mar-2023 11:06:04.102 20-Mar-2023 11:06:04.104
  3 Comments
Steven Lord
Steven Lord on 20 Mar 2023
I wouldn't expect you to need to include an extra hour in the data. Perhaps this is a time zone effect? Or maybe Daylight Savings Time?
Pablo Armañac Julián
Pablo Armañac Julián on 20 Mar 2023
Yes, now everything makes sense and 'TimeZone' works
added: " 'TimeZone','Europe/Madrid' " , instead of "+hours(1)"
TimestampSync_FormattedUnix_CAL = datetime ( TimestampSync_Unix_CAL./1000 , 'convertfrom','posixtime' , 'TimeZone','Europe/Madrid' , 'Format','yyyy/MMM/dd HH:mm:ss.SSS') ;

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