Extend time parsing to before 1970

It turns out that we are starting to have users that have logs that go
back that far. It won't be common, but let's get it right anyway.

NOTE! With us now supporting dates earlier in 1900, this also makes
"utc_mktime()" always add the "1900" to the year field.  That way we
avoid ever using the fairly ambiguous two-digit shorthand.

It didn't use to be all that ambiguous when we knew that any two-digit
number less than 70 had to be 2000+.  Now that we support going back to
earlier in the last centiry, that certainty is eroding.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

1 files changed, 54 insertions, 18 deletions

diff --git a/core/time.c b/core/time.c
index 0893f19d8..9992658b6 100644
--- a/core/time.c
+++ b/core/time.c
@@ -2,6 +2,24 @@
 #include "dive.h"
 
 /*
+ * The date handling internally works in seconds since
+ * Jan 1, 1900. That avoids negative numbers which avoids
+ * some silly problems.
+ *
+ * But we then use the same base epoch base (Jan 1, 1970)
+ * that POSIX uses, so that we can use the normal date
+ * handling functions for getting current time etc.
+ *
+ * There's 25567 dats from Jan 1, 1900 to Jan 1, 1970.
+ *
+ * NOTE! The SEC_PER_DAY is not so much because the
+ * number is complicated, as to make sure we always
+ * expand the type to "timestamp_t" in the arithmetic.
+ */
+#define SEC_PER_DAY  ((timestamp_t) 24*60*60)
+#define EPOCH_OFFSET (25567 * SEC_PER_DAY)
+
+/*
  * Convert 64-bit timestamp to 'struct tm' in UTC.
  *
  * On 32-bit machines, only do 64-bit arithmetic for the seconds
@@ -24,7 +42,14 @@ void utc_mkdate(timestamp_t timestamp, struct tm *tm)
 
 	memset(tm, 0, sizeof(*tm));
 
-	/* seconds since 1970 -> minutes since 1970 */
+	// Midnight at Jan 1, 1970 means "no date"
+	if (!timestamp)
+		return;
+
+	/* Convert to seconds since 1900 */
+	timestamp += EPOCH_OFFSET;
+
+	/* minutes since 1900 */
 	tm->tm_sec = timestamp % 60;
 	val = timestamp /= 60;
 
@@ -34,19 +59,21 @@ void utc_mkdate(timestamp_t timestamp, struct tm *tm)
 	tm->tm_hour = val % 24;
 	val /= 24;
 
-	/* Jan 1, 1970 was a Thursday (tm_wday=4) */
-	tm->tm_wday = (val + 4) % 7;
+	/* Jan 1, 1900 was a Monday (tm_wday=1) */
+	tm->tm_wday = (val + 1) % 7;
 
 	/*
-	 * Now we're in "days since Jan 1, 1970". To make things easier,
-	 * let's make it "days since Jan 1, 1968", since that's a leap-year
+	 * Now we're in "days since Jan 1, 1900". To make things easier,
+	 * let's make it "days since Jan 1, 1904", since that's a leap-year.
+	 * 1900 itself was not. The following logic will get 1900-1903
+	 * wrong. If you were diving back then, you're kind of screwed.
 	 */
-	val += 365 + 366;
+	val -= 365*4;
 
 	/* This only works up until 2099 (2100 isn't a leap-year) */
 	leapyears = val / (365 * 4 + 1);
 	val %= (365 * 4 + 1);
-	tm->tm_year = 68 + leapyears * 4;
+	tm->tm_year = 1904 + leapyears * 4;
 
 	/* Handle the leap-year itself */
 	mp = mdays_leap;
@@ -75,24 +102,33 @@ timestamp_t utc_mktime(struct tm *tm)
 	int year = tm->tm_year;
 	int month = tm->tm_mon;
 	int day = tm->tm_mday;
+	int days_since_1900;
+	timestamp_t when;
 
 	/* First normalize relative to 1900 */
-	if (year < 70)
+	if (year < 50)
 		year += 100;
 	else if (year > 1900)
 		year -= 1900;
 
-	/* Normalized to Jan 1, 1970: unix time */
-	year -= 70;
-
-	if (year < 0 || year > 129) /* algo only works for 1970-2099 */
-		return -1;
+	if (year < 0 || year > 129) /* algo only works for 1900-2099 */
+		return 0;
 	if (month < 0 || month > 11) /* array bounds */
-		return -1;
-	if (month < 2 || (year + 2) % 4)
+		return 0;
+	if (month < 2 || (year && year % 4))
 		day--;
 	if (tm->tm_hour < 0 || tm->tm_min < 0 || tm->tm_sec < 0)
-		return -1;
-	return (year * 365 + (year + 1) / 4 + mdays[month] + day) * 24 * 60 * 60UL +
-	       tm->tm_hour * 60 * 60 + tm->tm_min * 60 + tm->tm_sec;
+		return 0;
+
+	/* This works until 2099 */
+	days_since_1900 = year * 365 + (year - 1) / 4;
+
+	/* Note the 'day' fixup for non-leapyears above */
+	days_since_1900 += mdays[month] + day;
+
+	/* Now add it all up, making sure to do this part in "timestamp_t" */
+	when = days_since_1900 * SEC_PER_DAY;
+	when += tm->tm_hour * 60 * 60 + tm->tm_min * 60 + tm->tm_sec;
+
+	return when - EPOCH_OFFSET;
 }