# How to convert the number of seconds from an epoch into UTC using the datetime function.

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Matthew Casiano il 30 Lug 2021
Risposto: Sarthak il 13 Mar 2024
I am struggling to convert the number of seconds from an epoch (1980-01-06 GPS epoch) into UTC using the datetime function. The following commands give me a time of 18-Mar-2021 20:37:31, however in UTC this should be 18-Mar-2021 20:37:13 since I would need to subtract out the number of leap seconds from the total (currently there are 18 leap seconds since the 1980-01-06 GPS epoch).
NoOfSeconds=1300135051;
datetime(NoOfSeconds,'convertfrom','epochtime','Epoch','1980-01-06')
Can the datetime function automatically account for leapseconds in this conversion? I tried using the 'TimeZone','UTCLeapSeconds' name/value pair, but this adds 18 seconds since it assumes the time is already in UTC.
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### Risposta accettata

Peter Perkins il 2 Ago 2021
Matthew, this is easier than you think. In MATLAB, 'TimeZone','UTC' is sort of a ficticious timeline that pretends that leaps seconds don't exist (because hardly anyone wants to actually account for leap seconds). 'UTCLeapSeconds' is the timeline that knows all about leap seconds. The GPS timeline (in the real world) ticks off the same leap seconds as 'UTCLeapSeconds' in MATLAB does, it just doesn't treat them specially. So for the purposes of arithmetic, i.e. elapsed time, 'UTCLeapSeconds' is what you want. Then all you have to do is change to 'UTC' (or not, it's up to you).
>> epoch = datetime(1980,1,6,'TimeZone','UTCLeapSeconds')
epoch =
datetime
1980-01-06T00:00:00.000Z
>> dtUTC = epoch + seconds(1300135051)
dtUTC =
datetime
2021-03-18T20:37:13.000Z
>> dtUTC.TimeZone = 'UTC'
dtUTC =
datetime
18-Mar-2021 20:37:13
If I may ask, I would be interested in hearing more about how you work with GPS timestamps. Do you always get "seconds since 1980" as your input? Do you need to work with "week number, seconds within week"? Do you need to work with "human readable timestamps" (e.g. 18-Mar-2021 20:37:31) that refer to the GPS timeline (as opposed to UTC)? Do you need to convert back to one of those GPS time formats? Thanks in advance for anything you are able to share.
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Abhishek Ballaney il 20 Giu 2022
How to convert week, seconds to UTC?
Peter Perkins il 20 Giu 2022
I imagine what you mean is convert GPS week/second to UTC. It would be good for you to be clear.
The GPS timeline has no leap seconds or DST, so each week is equal length. Convert to seconds, and then do what's described earlier in this thread.

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### Più risposte (2)

Matthew Casiano il 31 Lug 2021
Since there is a leapseconds table included in Matlab, there is a workaround to obtain the cumalative number of leap seconds. This can then be subtracted off of the total number of seconds from an epoch.
% Determine the number of leap seconds since the 06-Jan-1980 GPS epoch by using a Matlab-defined timetable array called 'leapseconds'.
TimeTableLeapSeconds=leapseconds; % Matlab-defined timetable array of leap seconds information
CurrentDateTime=char(datetime('now')); % converts the current time to a character array
DateTimeRange = timerange('06-Jan-1980 00:00:00',CurrentDateTime); % creates a timerange object with the desired time range
[tf, whichrows]=containsrange(TimeTableLeapSeconds,DateTimeRange); % determines which rows of the leap seconds timetable are within the time range
AllRowsRange=TimeTableLeapSeconds(whichrows,1); % selects the first column of all rows in the range which is a timetable containing the leap second sign
AllLeapSecondSignsRange=char(AllRowsRange.Type); % convert timetable array to character array containing all the leap second signs within the time range
AllLeapSecondsRange=str2double(AllLeapSecondSignsRange+"1"); % append '1' to the sign and create a numeric array with all the leap seconds within the time range
CumLeapSecondAdj=sum(AllLeapSecondsRange); % sum contents of the numeric array producting the cumulative leap second adjustment
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Sarthak il 13 Mar 2024
function date = gps2utc(g0, g1)
date = datevec(datetime(g0*604800+g1,'convertfrom','epochtime','Epoch','1980-01-06'));
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