Fix off by one error in leap years before year 2000, and bad day names

Release/src/utilities/asyncrt_utils.cpp

@@ -639,26 +639,22 @@ static const int64_t SecondsFrom1900To2001 = INT64_C(3187296000);
 
 static const int64_t NtTo1900OffsetInterval = INT64_C(0x014F373BFDE04000);
 
-static int count_leap_years(int yearsSince1900)
+static int count_leap_years(const int yearsSince1900)
 {
-    int result = 0;
-    if (yearsSince1900 > 101)
-    {
-        result += 25;
-        yearsSince1900 -= 101;
-    }
+    int tmpYears = yearsSince1900 + 299; // shift into 1601, the first 400 year cycle including 1900
 
-    int year400 = yearsSince1900 / 400;
-    yearsSince1900 -= year400 * 400;
-    result += year400 * 97;
+    int year400 = tmpYears / 400;
+    tmpYears -= year400 * 400;
+    int result = year400 * 97;
 
-    int year100 = yearsSince1900 / 100;
-    yearsSince1900 -= year100 * 100;
+    int year100 = tmpYears / 100;
+    tmpYears -= year100 * 100;
     result += year100 * 24;
 
-    int year4 = yearsSince1900 / 4;
-    yearsSince1900 -= year4 * 4;
-    result += year4;
+    result += tmpYears / 4;
+
+    // subtract off leap years from 1601
+    result -= 72;
 
     return result;
 }
@@ -724,16 +720,11 @@ struct compute_year_result
     int secondsLeftThisYear;
 };
 
+static const int64_t secondsFrom1601To1900 = INT64_C(9435484800);
+
 static compute_year_result compute_year(int64_t secondsSince1900)
 {
-    int year = 0;
-    int64_t secondsLeft = secondsSince1900;
-    if (secondsSince1900 >= SecondsFrom1900To2001)
-    {
-        // After year 2001, shift there and start normal 400 year cycle
-        year += 101;
-        secondsLeft -= SecondsFrom1900To2001;
-    }
+    int64_t secondsLeft = secondsSince1900 + secondsFrom1601To1900; // shift to start of this 400 year cycle
 
     int year400 = static_cast<int>(secondsLeft / SecondsIn400Years);
     secondsLeft -= year400 * SecondsIn400Years;
@@ -747,8 +738,8 @@ static compute_year_result compute_year(int64_t secondsSince1900)
     int year1 = secondsInt / SecondsInYear;
     secondsInt -= year1 * SecondsInYear;
 
-    year += year400 * 400 + year100 * 100 + year4 * 4 + year1;
-    return {year, secondsInt};
+    // shift back to 1900 base from 1601:
+    return {year400 * 400 + year100 * 100 + year4 * 4 + year1 - 299, secondsInt};
 }
 
 utility::string_t datetime::to_string(date_format format) const
@@ -779,7 +770,7 @@ utility::string_t datetime::to_string(date_format format) const
     }
 
     const auto monthDay = yearDay - monthTable[month] + 1;
-    const auto weekday = static_cast<int>((secondsSince1900 / SecondsInDay + 3) % 7);
+    const auto weekday = static_cast<int>((secondsSince1900 / SecondsInDay + 1) % 7);
 
     char outBuffer[38]; // Thu, 01 Jan 1970 00:00:00 GMT\0
                         // 1970-01-01T00:00:00.1234567Z\0

Release/tests/functional/utils/datetime.cpp

@@ -10,6 +10,8 @@
  ****/
 
 #include "stdafx.h"
+#include <stdint.h>
+#include <string>
 
 using namespace utility;
 
@@ -131,6 +133,13 @@ SUITE(datetime)
         TestDateTimeRoundtrip(_XPLATSTR("9999-12-31T23:59:59Z"));
     }
 
+    TEST(emitting_time_correct_day) {
+        const auto test = utility::datetime() + UINT64_C(132004507640000000); // 2019-04-22T23:52:44 is a Monday
+        const auto actual = test.to_string(utility::datetime::RFC_1123);
+        const utility::string_t expected(_XPLATSTR("Mon"));
+        VERIFY_ARE_EQUAL(actual.substr(0, 3), expected);
+    }
+
     void TestRfc1123IsTimeT(const utility::char_t* str, uint64_t t)
     {
         datetime dt = datetime::from_string(str, utility::datetime::RFC_1123);