Managing how we record and use the current time and date.

§1. Clock. From the local environment, we'll extract the time at which we're running. 

    time_t right_now;
    struct tm *the_present = NULL;
    int fix_time_mode = FALSE;

    void Time::begin(void) {
        time_t right_now = time(NULL);
        the_present = localtime(&right_now);
        fix_time_mode = FALSE;
    }

The function Time::begin is used in 1/fm (§8).

§2. The command line option -fixtime causes any tool compiled with Foundation to fix the date as 11 a.m. on 28 March 2016, which is Inform's birthday. This makes it easier to automate testing, since we can compare output generated in one session with output generated another, even though that was on two different dates. 

    void Time::fix(void) {
        struct tm start;
        start.tm_sec = 0; start.tm_min = 0; start.tm_hour = 11;
        start.tm_mday = 28; start.tm_mon = 3; start.tm_year = 116; start.tm_isdst = -1;
        time_t pretend_time = mktime(&start);
        the_present = localtime(&pretend_time);
        fix_time_mode = TRUE;
    }

    int Time::fixed(void) {
        return fix_time_mode;
    }

The function Time::fix is used in 3/cla (§13.1).

The function Time::fixed appears nowhere else.

§3. Calendrical. The date of Easter depends on who and where you are. Inform's notional home is England. Following League of Nations advice in 1926, Easter is legally celebrated in England on the Sunday after the second Saturday in April, unless the Church requests otherwise. Since the Church has made this request every year since 1926 and shows no sign of coming around, we instead have to turn to church law, which is where it becomes complicated. There are five main algorithms ordained by major Christian churches: Catholic, continental European Protestant, Church of England, Eastern and Russian Orthodox. The first three always agree on the date, but usually disagree with the last two. The two eastern algorithms only disagree with each other once or twice a century, but the usual result has been riots with significant loss of life.

The official Church of England algorithm is a clumsy one adopted during the reign of George II. It was then thought important to use a non-Catholic method of calculation even though the same answer was required. We'll instead follow the algorithm of J.-M. Oudin, first published in the Bulletin astronomique in 1940, as adapted by the US Naval Observatory. Oudin corrected a small mistake in the calculation by Gauss (1800) of the Allgemeiner Reichskalender (1776) which reconciled Lutheran Easter with Gregorian, which in turn followed the reforms of Clavius et al. (1582), which in turn... and so on. See Leofranc Holford-Strevens, "The History of Time" (Oxford, 2005).

In principle we calculate the first Sunday after the first ecclesiastical moon that occurs on or after March 21. An "ecclesiastical moon" is one as seen from a longitude near Rome, except that the ratios used to adjust lunar and solar calendars are not quite right. The result is also tampered with to stop Easter from coinciding with the pagan anniversary of the founding of Rome (for the convenience of people living in the Vatican) and also to stop it from coinciding with the Jewish Passover (a change motivated purely by anti-Semitism). However, since they botched this tampering, it sometimes does.

Knuth remarks that calculating Easter was almost the only algorithmic research in the West for many centuries. Nevertheless the result is practically a random-number generator. The one thing to be said in its favour is that it can be computed accurately with integer arithmetic using fairly low numbers, and this we now do. 

    define CHRISTMAS_FEAST 1
    define EASTER_FEAST 2
    define NON_FEAST 3

    int Time::feast(void) {
        int this_month = the_present->tm_mon + 1;
        int this_day = the_present->tm_mday;
        int this_year = the_present->tm_year + 1900;

        int c, y, k, i, n, j, l, m, d;
        y = this_year;
        c = y/100;
        n = y-19*(y/19);
        k = (c-17)/25;
        i = c-c/4-(c-k)/3+19*n+15;
        i = i-30*(i/30);
        i = i-(i/28)*(1-(i/28)*(29/(i+1))*((21-n)/11));
        j = y+y/4+i+2-c+c/4;
        j = j-7*(j/7);
        l = i-j;
        m = 3+(l+40)/44;
        d = l+28-31*(m/4);

        if ((this_month == m) && (this_day >= d-2) && (this_day <= d+1))
            return EASTER_FEAST;  that is, Good Friday to Easter Monday
        if ((this_year == 2018) && (this_month == 3) && (this_day >= 30))
            return EASTER_FEAST;  Easter Sunday falls on 1 April in 2018

        if ((this_month == 12) && (this_day >= 25))
            return CHRISTMAS_FEAST;  that is, Christmas Day to New Year's Eve

        return NON_FEAST;
    }

The function Time::feast appears nowhere else.