MATLAB does not use any particular calendar system, such as the Julian or Gregorian calendar.
Please refer the documentation on the command DATENUM for a detailed description on how MATLAB calculates the date. You can use either 'help datenum' or 'doc datenum' in MATLAB command prompt to get detailed information on this command.
For MATLAB, the time functions begin with the CLOCK function, which returns a six-element vector:
C = [year month day hour minutes seconds]
For example, if you have the following code:
C = [2001 5 21 15 19 29];
DATESTR reformats this to:
datestr(C)
= 21-May-2001 15:19:29
DATENUM is a way of summarizing the six clock components in one double precision number (with an an accuracy of about 1.0e-5 seconds).
T = datenum(C)
which returns 730992.6385300926.
So, datenum(C) = 0 corresponds to C = [0 0 0 0 0 0], which is "day 0 of month 0 of year 0". Of course, as far as we know, no one has ever used such a calendar, but it would be the logical extrapolation of our current calendar back to times around year 0 A.D.
The Julian day number, JD, and MATLAB's datenum, T, are measuring the same thing -- number of days (and fractions of days) since some arbitrary ancient origin. One is simply an offset of the other. The Julian day number starts at noon, while the MATLAB datenum starts at midnight, so the offset involves half a day.
Here is the formula, applied to the time when you want to write this:
JD = T + 1721058.5
This returns 2452051.138530092. If you want to ignore the fraction, use the following code:
fix(JD)
which will return 2452051.
