Returns the number of days between this day and another day.
other - the other day
the number of days that this day is away from
the other (>0 if this day comes later)
private static int toJulian(int year,
Computes the Julian day number of the given day.
year - a year
month - a month
date - a day of the month
The Julian day number that begins at noon of
the given day
Positive year signifies CE, negative year BCE.
Remember that the year after 1 BCE was 1 CE.
A convenient reference point is that May 23, 1968 noon
is Julian day number 2440000.
Julian day number 0 is a Monday.
This algorithm is from Press et al., Numerical Recipes
in C, 2nd ed., Cambridge University Press 1992