posix/unix time to datetime

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

Answer 1

[Stephen23]

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

[Pablo Armañac Julián]

Perfect, thanks Stephen!

datetime(V/1000 , ... ) works perfect

FYI, I added "+ hours(1);" to make the two vectors exactly the same

[Steven Lord]

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]

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') ;