usr: ntpdate + date - the Game Boy knows what day it isHEADmaster

An MBC5 cart has no RTC, so SNTP is the only route to a wall clock. ntpdate
does one 48-byte UDP exchange on port 123 and hands the stamp to the kernel;
SYS_SETTIME keeps it with the tick it arrived at, and SYS_GETTIME adds the
elapsed ticks back, so date(1) is just formatting.

    /# ntpdate
    ntpdate: clock set from pool.ntp.org
    /# date
    Sep 24 21:01:23 UTC 2026        (host said 21:01:26)

The clock does not survive a power cycle - which is exactly what /rc is for:
`ntpdate` there and the machine boots knowing the time.

All of it is bounded subtraction, no long division or shifts: SDCC pulls those
from sm83.lib, whose modules link into the $A000 RAM window. The month tables
are string literals for the same family of reason - an initialized array goes
to _INITIALIZED at $A000 and the image overflows its bank ("buffer too small"
out of makebin), which is how this first failed to link.

1 files changed, 87 insertions, 0 deletions

diff --git a/usr/date.c b/usr/date.c
new file mode 100644
index 0000000..57108ab
--- /dev/null
+++ b/usr/date.c
@@ -0,0 +1,87 @@
+#include "gbos.h"
+/* date: print the wall clock, UTC.
+ *
+ * The kernel hands back unix seconds (SYS_GETTIME); turning that into a date
+ * is all bounded subtraction - no long division or shifts, which SDCC would
+ * pull out of sm83.lib and link into the $A000 RAM window (see usr/uptime.c
+ * and usr/build.sh). ~20k rounds for the day loop is a fraction of a second.
+ *
+ * Says so plainly when the clock has never been set: run ntpdate first. */
+
+/* No initialized arrays: SDCC puts those in _INITIALIZED at $A000, the RAM
+   window, and the image then overflows its bank ("buffer too small" out of
+   makebin - see usr/ansi.c and usr/build.sh). String literals live in _CODE,
+   so the tables are literals indexed by hand. */
+#define MDAYS "\x1f\x1c\x1f\x1e\x1f\x1e\x1f\x1f\x1e\x1f\x1e\x1f"
+#define MNAME "JanFebMarAprMayJunJulAugSepOctNovDec"
+
+static void two(unsigned int v) {
+    if (v < 10) putc('0');
+    putu(v);
+}
+static unsigned char leap(unsigned int y) {
+    if (y & 3) return 0;
+    if (y == 2100) return 0; /* good enough until then */
+    return 1;
+}
+
+void main(void) {
+    unsigned char t[4];
+    union {
+        unsigned long l;
+        unsigned char b[4];
+    } u;
+    unsigned long sec;
+    unsigned int days = 0, year = 1970, h, m, i, dim;
+
+    if (!gettime(t)) {
+        puts("date: clock not set (run ntpdate)");
+        nl();
+        sexit(1);
+    }
+    u.b[0] = t[0];
+    u.b[1] = t[1];
+    u.b[2] = t[2];
+    u.b[3] = t[3];
+    sec = u.l;
+
+    while (sec >= 86400UL) {
+        sec -= 86400UL;
+        days++;
+    }
+    h = 0;
+    while (sec >= 3600UL) {
+        sec -= 3600UL;
+        h++;
+    }
+    m = 0;
+    while (sec >= 60UL) {
+        sec -= 60UL;
+        m++;
+    }
+
+    for (;;) { /* years, then months */
+        unsigned int len = leap(year) ? 366 : 365;
+        if (days < len) break;
+        days -= len;
+        year++;
+    }
+    for (i = 0; i < 12; i++) {
+        dim = (unsigned char)MDAYS[i];
+        if (i == 1 && leap(year)) dim = 29;
+        if (days < dim) break;
+        days -= dim;
+    }
+    writes(MNAME + i * 3, 3);
+    putc(' ');
+    putu(days + 1);
+    putc(' ');
+    two(h);
+    putc(':');
+    two(m);
+    putc(':');
+    two((unsigned int)sec);
+    puts(" UTC ");
+    putu(year);
+    nl();
+}