How to change time stamp to readable date and time without giving reference time ?

Below code is to generate Date and Time from a timeStamp but with the help of reference date and time (refTime). However , I need to generate date and time without giving reference time. How can i do that? Thanks in advance.

   refTime = [2016,08,02,10,28,52];
   refNum = datenum(refTime);
   % refStr = datestr(refNum,'yyyy-mm-dd HH:MM:SS.FFF');
   Dat_timeStamp = [1.454958517009081e+18,1.454958517014576e+18, 1.454958517018680e+18];
   % Processing data
   % dn = 24*60*60*1000*1000; % Microseconds! I have changed this equation to many options but nothing was helpful
   for i = 1 : size(Dat_timeStamp,2)-1
        usecs = double(Dat_timeStamp(1,i+1)) - double(Dat_timeStamp(1,1));
        secsDiff = usecs / 1e9;
        matDateTime = refNum + secsDiff / (24*60*60);
        gzStr{i,1} = datestr(matDateTime,'yyyy-mm-dd HH:MM:SS'); % I can't use 'SS.FFFFFF'
    end;

[dpb]

What's the definition for the time stamp??? If, as it appears to be, it is some number of milli- or micro-seconds from some time as is the common definition, there's no way to convert to absolute without knowing the reference point.

[Jan]

I don't get it. What are the inputs and what are the wanted outputs?

[Naveed Khan]

hank you for your answer, Its almost what i want, but my timestamp is in 19 digits. i.e. in nanoseconds, so should i also divide it by 1000, or by 1000000000"

Answer 1

[Walter Roberson]

Are you trying to deal with POSIX times? NTP64 times?

http://www.mathworks.com/matlabcentral/answers/228436-how-convert-13-digits-of-timestamp#answer_185339

[Naveed Khan]

Thank you for your answer, yes it is almost the same. But the problem is that his answer is for timeStamp of size 13, whereas i have 19 digits timestamp. So it does not give me correct answers. Even Matlab datestr() gives error. I also tried belwo procedure, but their is an error in the answer, may be because 1970 is unix reference year, whereas i am using Windows. So how can i fix this. Answer of code vs origional is given below.

   TS = 1.454956105656222e+18;
   datestr(datevec(TS/60/60/24/1000000000) + [1970 0 0 0 0 0])
   ans =
   06-Feb-2016 18:28:25

Whereas origional output should be

    08-Feb-2016 10:28:52

So where i am doing error?

[Steven Lord]

You're expecting a number that would require nineteen decimal places to store to be exactly represented in IEEE double precision.

TS = 1.454956105656222e+18;
delta = eps(TS) % = 256

delta is the distance between TS and the next exactly representable double precision number.

TS2 = TS+delta/4;
isequal(TS, TS2) % true!

So is your "time stamp" TS nanoseconds, TS + 64 nanoseconds, or some number of nanoseconds inbetween? Each of those would be represented by the TS you posted. This may not be a big deal for your scenario, but given that you're using nanosecond precision I bet it is.

One solution that might work would be to switch from double to uint64.

>> [uint64(TS); uint64(1454956105656222483); intmax('uint64')]

ans =

   1454956105656221952
   1454956105656222483
  18446744073709551615

[Walter Roberson]

If you need nanoseconds then you need to switch to using datetime objects (R2014b or later).

The 8 hour time difference between what you get and what you expect might be related to timezone factors, perhaps. datestr() reports local time. Would you happen to be in Australia, or in Indonesia perhaps?

[dpb]

"The 8 hour time difference between what you get and what you expect might be related to timezone..."

But he's also got 27 second difference as well so between 18:28:25 and 18:28:52 so looks like losing precision is also hurting...altho that seems a lot I didn't try to compute at what position that would be...

[Naveed Khan]

Thanks for your detailed answer. Yes the dataset is collected in US in february 2016 and the time zone is -8. but currently i am in United Kingdom (UK). Please guide me that how to remove this 8 hours difference?

[Walter Roberson]

You can create datetime objects and attach a timezone, if you have R2014b or later.

[Naveed Khan]

Thanks for the reply , yes i have R2014b. Can you kindly give some hints taking above as an example that i already mention, can you use that time stamp and generate actual time and date by creating datetime objects and attached a timezone?

[Walter Roberson]

http://www.mathworks.com/help/matlab/matlab_prog/specify-time-zones.html

[Naveed Khan]

Thanks Walter Roberson for your guidance and help. Finally i get the required output. once again thanks .....:)

Answer 2

[Peter Perkins]

When I do this

>> datetime('08-Feb-2016 10:28:52','Format','dd-MMM-yyyy HH:mm:ss.SSSSSSSSS') - seconds(1.454956105656222e+9)
ans = 
   31-Dec-1969 16:00:26.343777832

It makes me suspect that the issue is not only time zones, not only floating point round-off, but also leap seconds.