nill - 3 months ago 24

Java Question

Alright so a bit of a weird problem here... I need to find the day of the week (i.e. Monday, Tuesday...) given MM-DD-YYYY. So basically what java calendar would do, but without using java calendar.

Answer

It is possible, though unusual, to compute a number that corresponds to the day of the week from a calendar date.

In brief, you will first need to calculate a serial date number from the calendar date, i.e. a number that is a continuous count of days that have elapsed since a certain fixed point in time (informally called 'the epoch'). The most commonly encountered serial date scheme encountered in modern computing is Posix Time, which has an epoch date of Jan 1, 1970 at midnight UTC.

You will need to decide what level of precision is needed for this calculation, eg. whether you will need to account for the Julian Calendar (used in most of Europe before the Gregorian Calendar reform by Pope Gregory in 1584), whether to correct for century days, etc.

Several algorithms are available to arithmetically convert a calendar date to a serial date number with a given epoch (historically, the most commonly used epoch for these calculations has been the Julian Day Number -- not to be confused with the Julian Calendar -- system which counts days from November 24, 4714 BC, in the proleptic Gregorian calendar). Below is Java code which implements one such algorithm published by Jean Meeus in his book "Astronomical Algorithms, 2nd Ed." This algorithm assumes that days are exactly 86400 seconds in length, accounts for the general Gregorian Reform, and accounts for century and leap days:

```
public class JulianDay {
private static final int YEAR = 0;
private static final int MONTH = 1;
private static final int DAY = 2;
private static final int HOURS = 3;
private static final int MINUTES = 4;
private static final int SECONDS = 5;
private static final int MILLIS = 6;
:
:
// Converts a timestamp presented as an array of integers in the following
// order (from index 0 to 6): year,month,day,hours,minutes,seconds,millis
// month (1-12), day (1-28 or 29), hours (0-23), min/sec (0-59) to a
// Julian Day Number.
// For clarity and simplicity, the input values are assumed to be well-formed;
// error checking is not implemented in the snippet.
public static double toJD(int[] ymd_hms) {
int y = ymd_hms[YEAR];
int m = ymd_hms[MONTH];
double d = (double) ymd_hms[DAY];
d = d + ((ymd_hms[HOURS] / 24.0) +
(ymd_hms[MINUTES] / 1440.0) +
(ymd_hms[SECONDS] / 86400.0) +
(ymd_hms[MILLIS] / 86400000.0));
if (m == 1 || m == 2) {
y--;
m = m + 12;
}
double a = Math.floor(y / 100);
double b = 2 - a + Math.floor(a / 4);
return (Math.floor(365.25 * (y + 4716.0)) +
Math.floor(30.6001 * (m + 1)) +
d + b - 1524.5);
}
}
```

Once you have a serial date number, it is straightforward to compute the day of the week from the remainder when the date number is divided by 7 (the number of days in a week).