Convert Date and Time to Julian Date or POSIX Time
You can convert datetime
arrays to represent points in time in specialized numeric formats. In general, these formats represent a point in time as the number of seconds or days that have elapsed since a specified starting point. For example, the Julian date is the number of days and fractional days that have elapsed since the beginning of the Julian period. The POSIX® time is the number of seconds that have elapsed since 00:00:00 1-Jan-1970 UTC (Coordinated Universal Time). MATLAB® provides the juliandate
and posixtime
functions to convert datetime
arrays to Julian dates and POSIX times.
While datetime
arrays are not required to have a time zone, converting "unzoned" datetime
values to Julian dates or POSIX times can lead to unexpected results. To ensure the expected result, specify the time zone before conversion.
Specify Time Zone Before Conversion
You can specify a time zone for a datetime
array, but you are not required to do so. In fact, by default the datetime
function creates an "unzoned" datetime
array.
Create a datetime
value for the current date and time.
d = datetime("now")
d = datetime
20-Jul-2024 14:47:29
d
is constructed from the local time on your machine and has no time zone associated with it. In many contexts, you might assume that you can treat the times in an unzoned datetime
array as local times. However, the juliandate
and posixtime
functions treat the times in unzoned datetime
arrays as UTC times, not local times. To avoid any ambiguity, it is recommended that you avoid using juliandate
and posixtime
on unzoned datetime
arrays. For example, avoid using posixtime(datetime("now"))
in your code.
If your datetime
array has values that do not represent UTC times, specify the time zone using the TimeZone
name-value pair argument so that juliandate
and posixtime
interpret the datetime
values correctly.
d = datetime("now","TimeZone","America/New_York")
d = datetime
20-Jul-2024 14:47:29
As an alternative, you can specify the TimeZone
property after you create the array.
d.TimeZone = "America/Los_Angeles"
d = datetime
20-Jul-2024 11:47:29
To see a complete list of time zones, use the timezones
function.
Convert Zoned and Unzoned Datetime Values to Julian Dates
A Julian date is the number of days (including fractional days) since noon on November 24, 4714 BCE, in the proleptic Gregorian calendar, or January 1, 4713 BCE, in the proleptic Julian calendar. To convert datetime
arrays to Julian dates, use the juliandate
function.
Create a datetime
array and specify its time zone.
DZ = datetime("2016-07-29 10:05:24") + calmonths(1:3); DZ.TimeZone = "America/New_York"
DZ = 1x3 datetime
29-Aug-2016 10:05:24 29-Sep-2016 10:05:24 29-Oct-2016 10:05:24
Convert D
to the equivalent Julian dates.
format longG
JDZ = juliandate(DZ)
JDZ = 1×3
2457630.08708333 2457661.08708333 2457691.08708333
Create an unzoned copy of DZ
. Convert D
to the equivalent Julian dates. As D
has no time zone, juliandate
treats the times as UTC times.
D = DZ;
D.TimeZone = "";
JD = juliandate(D)
JD = 1×3
2457629.92041667 2457660.92041667 2457690.92041667
Compare JDZ
and JD
. The differences are equal to the time zone offset between UTC and the America/New_York
time zone in fractional days.
JDZ - JD
ans = 1×3
0.166666666511446 0.166666666511446 0.166666666511446
Convert Zoned and Unzoned Datetime Values to POSIX Times
The POSIX time is the number of seconds (including fractional seconds) elapsed since 00:00:00 1-Jan-1970 UTC (Coordinated Universal Time), ignoring leap seconds. To convert datetime
arrays to POSIX times, use the posixtime
function.
Create a datetime
array and specify its time zone.
DZ = datetime("2016-07-29 10:05:24") + calmonths(1:3); DZ.TimeZone = "America/New_York"
DZ = 1x3 datetime
29-Aug-2016 10:05:24 29-Sep-2016 10:05:24 29-Oct-2016 10:05:24
Convert D
to the equivalent POSIX times.
PTZ = posixtime(DZ)
PTZ = 1×3
1472479524 1475157924 1477749924
Create an unzoned copy of DZ
. Convert D
to the equivalent POSIX times. As D
has no time zone, posixtime
treats the times as UTC times.
D = DZ;
D.TimeZone = "";
PT = posixtime(D)
PT = 1×3
1472465124 1475143524 1477735524
Compare PTZ
and PT
. The differences are equal to the time zone offset between UTC and the America/New_York
time zone in seconds.
PTZ - PT
ans = 1×3
14400 14400 14400
See Also
datetime
| timezones
| posixtime
| juliandate