How can I convert an epoch value to a 'datetime' value, with millisecond precision?

How can I transfer an epoch value into real time, with precision to the millisecond?

Answer 1

[MathWorks Support Team]

You can transfer an epoch value, 'epVal', to a 'datetime' value with millisecond precision with the following line:

>> d = datetime(epVal, 'convertfrom', 'posixtime', 'Format', 'MM/dd/yy HH:mm:ss.SSS');

For further information about 'datetime' values and their properties, please see the following MATLAB documentation page:

https://www.mathworks.com/help/matlab/ref/datetime.html

[Peter Perkins]

As described in the doc, the value you pass as 'ConvertFrom' for 'posixtime' is

"Number of seconds since 1-Jan-1970 00:00:00 UTC"

So if "with millisecond precision" means "seconds plus fractional seconds", the above is correct. "Seconds plus fractional seconds" is what

>> posixtime(datetime('now'))
ans =
          1553518031.88491

returns.

But often ""with millisecond precision"" means "whole milliseconds since 1970", and the above is not the right result. Instead:

>> dnow = datetime('now')
dnow = 
  datetime
   25-Mar-2019 12:48:59
>> et = milliseconds(dnow - datetime(1970,1,1))
et =
          1553518139117.12
>> datetime(et,'ConvertFrom','epochtime','TicksPerSecond',1000)
ans = 
  datetime
   25-Mar-2019 12:48:59