UTC Time to String Conversion - matlab

I am looking for helping doing time conversions from UTC time to string using MATLAB.
I am trying to extract time from a data file collected at the end of October 2010.
The data file says it is reporting in UTC time and the field is an integer string value in milliseconds that is around 3.02e11. I would like to convert this to a string but am have some trouble.
I figured out that the units are most definitely in milliseconds so I convert this to fractions of days to be compatible with datenum format.
If the data was collected at the end of October (say, October 31, 2010) then I can guess what kind of number I might get. I thought that January 1, 2001 would be a good epoch and calculated what sort of number (in days) I might get:
suspectedDate = datenum('October 31, 2010')
suspectedEpoch = datenum('January 1, 2001')
suspectedTimeInDays = suspectedDate - suspectedEpoch
Which comes out as 3590.
However, my actual time, in days, comes out with the following code
actualTime = 3.02e11
actualTimeInDays = 3.02e11/1000/24/3600
as 3495.4.
This is troubling as the difference is only 94.6 -- not a full year. This would mean either the documentation for the file is wrong or the epoch is close to April 1-5, 2001:
calculatedEpoch = suspectedDate - actualTimeInDays
calculatedEpochStr = datestr(calculatedEpoch)
Alternately, if the epoch is January 1, 2001 then the actual date in the file is from the end of July.
ifEpochIsJanuaryDate = suspectedEpoch + actualTimeInDays
ifEpochIsJanuaryDateStr = datestr(ifEpochIsJanuaryDate)
Is this a known UTC format and can anyone give suggestions on how to get an October date from 3.02e11 magnitude number?

Unix time today is about 13e11, and is measured in ms since 1970.
If your time is ~3e11, then it's probably since year 2000.
>> time_unix = 1339116554872; % example time
>> time_reference = datenum('1970', 'yyyy');
>> time_matlab = time_reference + time_unix / 8.64e7;
>> time_matlab_string = datestr(time_matlab, 'yyyymmdd HH:MM:SS.FFF')
time_matlab_string =
20120608 00:49:14.872
Notes:
1) change 1970 into 2000 if your time is since 2000;
2) See the definition of matlab's time.
3) 8.64e7 is number of milliseconds in a day.
4) Matlab does not apply any time-zone shifts, so the result is the same UTC time.
5) Example for backward transformation:
>> matlab_time = now;
>> unix_time = round(8.64e7 * (matlab_time - datenum('1970', 'yyyy')))
unix_time =
1339118367664

You can't just make up your own epoch. Also datenum returns things in days. So the closeness you got with doing your math was just a coincidence.
Turns out that
>> datenum('Jan-1-0000')
ans =
1
and
>> datenum('Jan-1-0001')
ans =
367
So Matlab should be returning things in days since Jan. 1, 0000. (Not a typo)
However, I'd look carefully at this 3.02e11 number and find out exactly what it means. I'm pretty sure it's not standard Unix UTC, which should be seconds since January 1, 1970. It's way too big. It's close to GMT: Mon, 1 Jan 11540 08:53:20 UTC.

Related

Extract time from date time and find difference between 2 times

I am trying to convert EPOC time to date time and need to extract the time only from that
I am doing below
$min = $Time_Start | measure -Minimum
$max = $Time_End | measure -Maximum
[datetime]$oUNIXDatemin=(Get-Date 01.01.1970)+([System.TimeSpan]::fromseconds($min.Minimum))
$oUNIXDatemin_1 = $oUNIXDatemin.ToString("HH:mm:ss")
[datetime]$oUNIXDatemax=(Get-Date 01.01.1970)+([System.TimeSpan]::fromseconds($max.Maximum))
$oUNIXDatemax_1 = $oUNIXDatemax.ToString("HH:mm:ss")
Problem is while converting I am getting $oUNIXDatemin_1 and $oUNIXDatemax_1 value like
$oUNIXDatemin_1
12 October 2021 07:46:46
$oUNIXDatemax_1
12 October 2021 21:16:04
My EPOC values are
$min.Minimum
1634024806
$max.Maximum
1634073364
Please let me know what is wrong here. Need to find the difference in HH:mm:ss format.
In PowerShell, you'd usually use a format string. Subtracting two PowerShell datetimes returns a value of type Timespan, which is well-behaved over a span of more than 24 hours.
([datetime]"12 October 2021 21:16:04" - [datetime]"12 October 2021 07:46:46") -f "HH:mm:ss"
13:29:18
Be careful here. Both intervals (durations) and time (of day) have the same format, but different meanings. For example, it makes sense to multiply the interval "01:00:00" (1 hour) by 3 to get three hours; it doesn't make sense to multiply the time "01:00:00" (1 o'clock AM) by 3.
I'm sure the overall calculation can be simplified, but it's too early for me.

How to convert formatted date to unix epoch in Libreoffice calc

I have column with cell format date or time (DD.MM.YYYY HH:MM:SS) and values like
03.12.2013 14:01:49
04.12.2013 10:19:27
04.12.2013 12:44:56
04.12.2013 14:20:12
04.12.2013 18:30:21
I need those values converted to unix epoch (seconds since 1970). Somehow it feels like the values are not recognized as dates, but rather as strings. I tried different formats, had little luck with dates without time.
Operations performed on date data should be automatic provided that the cells are formatted as as a user defined DD.MM.YYYY HH:MM:SS in the 'Format' > 'Cells' > 'Numbers' tab.
If you're using the standard settings, LibreOffice Calc uses 12/30/1899 as it's default date. So the first step is getting the number of days between 12/30/1899 and 1/1/1970:
=(DATE(1970,1,1) - DATE(1899,12,30)) = 25569
Number of seconds in a day:
=(60 * 60 * 24) = 86400
If, for example, in cell A2 you have the date 03.12.2013 14:01:49. I subtract the difference between Calc's default date and the Unix Epoch we just calculated, and multiply it by the number of seconds in a day:
=(A2 - 25569) * 86400
The result is a value of 1363096909 which is the Epoch time in seconds. If you need it in milliseconds, multiply the equation by 1000.
If it's something you use a lot, you can create a custom function that does this. Go to Tools > Macros > Edit Macros, and type out the following into whichever module comes up:
REM ***** BASIC *****
Function EPOCH(date_cell)
EPOCH = (date_cell - 25569)*86400
End Function
Close the macro IDE, and now you can use your EPOCH() like any other function!
This formula worked for me, where the others above did not:
= DATE( 1970, 1, 1 ) + ( A1 / 86400 )

How can I find the difference in hours between 2 times in character arrays?

I have 2 times stored in character arrays in MATLAB.
a = '11:00 PM'
b = '07:30 AM'
I want to find the difference in hours between the 2 times, which should be 8.5 hours in this example. Is there any short method to do that? I can datenum both numbers, subtract them, datevec the difference, extract the hours and minutes from the vector, and convert them into hours, but this takes a lot of lines. Is there a more efficient way of doing this or is there an existing function?
You can do this by converting each string using datetime, taking the difference, then converting the result with hours:
numHours = hours(diff(datetime({a; b}, 'InputFormat', 'hh:mm a')));
numHours = numHours + 24.*(numHours < 0)
numHours =
8.5000
The second line accounts for the condition in your example, where the second time has to occur on the next day for the time difference to be positive, so 24 hours are added to the (negative) difference.
add a date to the time
like
a = '1/1/2000 11:00 PM'
b = '1/1/2000 07:30 AM'
the convert the string to datetime
x=str2num(strrep(a,':',''))
y=str2num(strrep(b,':',''))
then fine the difference between 2 dates
e = etime(x,y)
this will give you number of seconds between both times

Octave database package timestamp from PostgreSQL

Octave database package determines that PostgreSQL timestamp is returned in Octave as 8-byte-time-value positive or negative difference to 2000-01-01 00:00 (int64 scalar, representing microseconds)
e.g. timestamp 2015-12-04 11:22:08 is returned as 502543328000000 in Octave.
How do I convert this to more readable and usable form if I intend to plot time series in Octave?
Use datestr. Note that datestr expects the serial date number in units of days and since 1 Jan 0000 as seen from the datenum docs
Return the date/time input as a serial day number, with Jan 1, 0000
defined as day 1.
So first we would have to convert your PostgreSQL serial number to (a) units of days instead of microseconds and (b) to start from the year 0000 instead of 2000.
timestamp = 502543328000000; % in microsecond since 2000
timestamp = timestamp / (1000000*60*60*24); % Days since 2000
timestamp = timestamp + datenum([2000,1,1]); % Days since 0000
datestr(timestamp, "yyyy-MM-dd HH:mm:ss") % Formatted string
There is a problem with datestr format in previous answer, i.e. it returns months in place of minutes and minutes in place of months. Proper format is probably
timestamp = input_time; % in microsecond since 2000
timestamp = timestamp ./ (1000000*60*60*24); % Days since 2000
timestamp = timestamp + datenum([2000,1,1]); % Days since 0000
datestr(timestamp(i), "yyyy-mm-dd HH:MM:SS.FFF") % Formatted string
where FFF stands for miliseconds if needed.

How to work with Unix timestamps in Matlab?

I have some data files with Unix timestamps (in this case, number of milliseconds since Jan 1, 1970 00:00 UTC). I would like to convert these to human-friendly date/time strings (e.g. 31-Aug-2012 11:36:24) in Matlab. Is there an easy way to do this in Matlab, or am I better off using an external library (e.g. java.text.SimpleDateFormat)?
How about
date = datestr(unix_time/86400 + datenum(1970,1,1))
if unix_time is given in seconds, unix_time/86400 will give the number of days since Jan. 1st 1970. Add to that the offset used by Matlab's datenum (datenum(0000,1,1) == 1), and you have the amount of days since Jan. 1st, 0000. This can be easily converted to human-readable form by Matlab's datestr.
If you have milliseconds, just use
date = datestr(unix_time/86400/1000 + datenum(1970,1,1))
Wrapped in functions, these would be
function dn = unixtime_to_datenum( unix_time )
dn = unix_time/86400 + 719529; %# == datenum(1970,1,1)
end
function dn = unixtime_in_ms_to_datenum( unix_time_ms )
dn = unix_time_ms/86400000 + 719529; %# == datenum(1970,1,1)
end
datestr( unixtime_to_datenum( unix_time ) )
Newer versions of MATLAB (verified in R2015a) have a datetime type that is useful for working with and formatting dates and times. You can convert UNIX timestamps into a MATLAB datetime with
dt = datetime( unix_time, 'ConvertFrom', 'posixtime' );
and then use datestr as before for formatting as a string
datestr( dt )