# Convert Julian Date + Time in GMT to calendar day + Time in EST

38 views (last 30 days)
Aaron J. Hendrickson on 13 Feb 2024
Commented: Siddharth Bhutiya on 22 Feb 2024
I am given three strings
year = '2024';
JDDD = 'J025'; % Julian date
GMT = '19:29:29.231' % GMT time
JDDD is the "Julian Date" specifying the number of days elapsed since Jan 1 of the current year. So 025 in JDDD corresponds to January 25th.
I would appreciate if anyone could show me how to convert this data using datetime into the format 'yyyy-MM-dd HH:mm:ss.SSS' with the time in EST.

Star Strider on 13 Feb 2024
Edited: Star Strider on 13 Feb 2024
This works —
syear = '2024';
JDDD = 'J025'; % Julian date
GMT = '19:29:29.231'; % GMT time
doy = sscanf(JDDD,'J%3d')
doy = 25
DTUCT = datetime(str2double(syear),1,doy) + timeofday(datetime(GMT));
DTUCT .TimeZone = 'UCT';
Warning: 'UCT' specifies a time zone with a fixed offset from UTC, +00:00. This zone does not follow daylight saving time, and so may give unexpected results. See the datetime.TimeZone property for details about specifying time zones.
DTUCT.Format = 'yyyy-MMM-dd HH:mm:ss.SSS'
DTUCT = datetime
2024-Jan-25 19:29:29.231
DTEST = DTUCT;
DTEST.TimeZone = 'America/New_York'
DTEST = datetime
2024-Jan-25 14:29:29.231
The datetime function will do the day-of-year conversion automatically. For example, if the day is 250 rather than 25, datetime returns:
Out = datetime(2024,1,250,'TimeZone','GMT') + timeofday(datetime(GMT))
Out = datetime
06-Sep-2024 19:29:29
.
Siddharth Bhutiya on 22 Feb 2024
You can do the conversion in one datetime call by using the "D" day of year format specifier and appending the timezone info in the string.
syear = '2024';
JDDD = 'J025'; % Julian date
GMT = '19:29:29.231'; % GMT time
dtStr = "" + syear + "-" + JDDD + " " + GMT + " GMT"
dtStr = "2024-J025 19:29:29.231 GMT"
datetime(dtStr,InputFormat="uuuu-'J'DDD HH:mm:ss.SSS z",TimeZone="America/New_York",Format="uuuu-MMM-dd HH:mm:ss.SSS")
ans = datetime
2024-Jan-25 14:29:29.231

Dyuman Joshi on 13 Feb 2024
Edited: Dyuman Joshi on 13 Feb 2024
Note that I have changed the inputs YEAR and JDDD to numeric values as they are easier to work with -
I changed the name to all capital letters as it clashed with the function year. Make sure to not use any inbuilt function names from MATLAB as names for variables/scripts.
%Inputs
YEAR = 2024;
JDDD = 25; % Julian date
GMT = '19:29:29.231'; % GMT time
%Get the date and convert to a char array
d1 = char(datetime(YEAR, 1, JDDD))
d1 = '25-Jan-2024'
%Concatenate the date with time
vec = sprintf('%s %s', d1, GMT)
vec = '25-Jan-2024 19:29:29.231'
%Make a datetime element specifying UTC as the time-zone
out = datetime(vec, 'InputFormat', 'dd-MMM-yyyy HH:mm:ss.SSS', 'TimeZone', 'UTC')
out = datetime
25-Jan-2024 19:29:29
%Convert the time zone to EST
out.TimeZone = 'America/Atikokan';
%Note that there are many different IANA zone IDs for EST timezones -
%America/Atikokan, America/Jamaica and America/Panama etc, Any of these can be used
out
out = datetime
25-Jan-2024 14:29:29
Dyuman Joshi on 13 Feb 2024
This works with any julian day from 1st January of the given year -
%Inputs
YEAR = 2024;
JDDD = 69; % Julian date
GMT = '19:29:29.231'; % GMT time
%Get the date and convert to a char array
d1 = char(datetime(YEAR, 1, JDDD))
d1 = '09-Mar-2024'
%Concatenate the date with time
vec = sprintf('%s %s', d1, GMT);
%Make a datetime element specifying UTC as the time-zone
out = datetime(vec, 'InputFormat', 'dd-MMM-yyyy HH:mm:ss.SSS', 'TimeZone', 'UTC');
%Convert the time zone to EST
out.TimeZone = 'America/Atikokan';
out
out = datetime
09-Mar-2024 14:29:29
Alexander on 13 Feb 2024
Even if it is very theoretical, not in 2100 ;-)

Les Beckham on 13 Feb 2024
It's not pretty, but it works.
yr = '2024'; %<<< renamed to avoid shadowing the year function
JDDD = 'J025'; % Julian date
GMT = '19:29:29.231' % GMT time
GMT = '19:29:29.231'
t = datetime(str2double(yr), 1, str2double(strrep(JDDD, 'J', '')), ...
str2double(GMT(1:2)), str2double(GMT(4:5)), str2double(GMT(7:end)), 'TimeZone', 'GMT')
t = datetime
25-Jan-2024 19:29:29
t.TimeZone = 'America/New_York'
t = datetime
25-Jan-2024 14:29:29
t.Format = 'yyyy-MM-dd HH:mm:ss.SSS'
t = datetime
2024-01-25 14:29:29.231
Note that this will work even if JDDD is more than 31.
JDDD = 'J035';
t = datetime(str2double(yr), 1, str2double(strrep(JDDD, 'J', '')), ...
str2double(GMT(1:2)), str2double(GMT(4:5)), str2double(GMT(7:end)), 'TimeZone', 'GMT');
t.TimeZone = 'America/New_York';
t.Format = 'yyyy-MM-dd HH:mm:ss.SSS'
t = datetime
2024-02-04 14:29:29.231

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