Fix crash resulting from bad month check in core_bind.cpp

Also, make it clear that day is 0-based. This might cause very slight differcies in existing games.
Fixes #18221

core/bind/core_bind.cpp

@@ -635,8 +635,8 @@ uint64_t _OS::get_unix_time_from_datetime(Dictionary datetime) const {
 	unsigned int second = ((datetime.has(SECOND_KEY)) ? static_cast<unsigned int>(datetime[SECOND_KEY]) : 0);
 	unsigned int minute = ((datetime.has(MINUTE_KEY)) ? static_cast<unsigned int>(datetime[MINUTE_KEY]) : 0);
 	unsigned int hour = ((datetime.has(HOUR_KEY)) ? static_cast<unsigned int>(datetime[HOUR_KEY]) : 0);
-	unsigned int day = ((datetime.has(DAY_KEY)) ? static_cast<unsigned int>(datetime[DAY_KEY]) : 0);
-	unsigned int month = ((datetime.has(MONTH_KEY)) ? static_cast<unsigned int>(datetime[MONTH_KEY]) - 1 : 0);
+	unsigned int day = ((datetime.has(DAY_KEY)) ? static_cast<unsigned int>(datetime[DAY_KEY]) : 1);
+	unsigned int month = ((datetime.has(MONTH_KEY)) ? static_cast<unsigned int>(datetime[MONTH_KEY]) : 1);
 	unsigned int year = ((datetime.has(YEAR_KEY)) ? static_cast<unsigned int>(datetime[YEAR_KEY]) : 0);
 
 	/// How many days come before each month (0-12)
@@ -656,15 +656,15 @@ uint64_t _OS::get_unix_time_from_datetime(Dictionary datetime) const {
 	ERR_EXPLAIN("Invalid hour value of: " + itos(hour));
 	ERR_FAIL_COND_V(hour > 23, 0);
 
-	ERR_EXPLAIN("Invalid month value of: " + itos(month + 1));
-	ERR_FAIL_COND_V(month + 1 > 12, 0);
+	ERR_EXPLAIN("Invalid month value of: " + itos(month));
+	ERR_FAIL_COND_V(month > 12 || month == 0, 0);
 
 	// Do this check after month is tested as valid
-	ERR_EXPLAIN("Invalid day value of: " + itos(day) + " which is larger than " + itos(MONTH_DAYS_TABLE[LEAPYEAR(year)][month]));
-	ERR_FAIL_COND_V(day > MONTH_DAYS_TABLE[LEAPYEAR(year)][month], 0);
+	ERR_EXPLAIN("Invalid day value of: " + itos(day) + " which is larger than " + itos(MONTH_DAYS_TABLE[LEAPYEAR(year)][month - 1]) + " or 0");
+	ERR_FAIL_COND_V(day > MONTH_DAYS_TABLE[LEAPYEAR(year)][month - 1] || day == 0, 0);
 
 	// Calculate all the seconds from months past in this year
-	uint64_t SECONDS_FROM_MONTHS_PAST_THIS_YEAR = DAYS_PAST_THIS_YEAR_TABLE[LEAPYEAR(year)][month] * SECONDS_PER_DAY;
+	uint64_t SECONDS_FROM_MONTHS_PAST_THIS_YEAR = DAYS_PAST_THIS_YEAR_TABLE[LEAPYEAR(year)][month - 1] * SECONDS_PER_DAY;
 
 	uint64_t SECONDS_FROM_YEARS_PAST = 0;
 	for (unsigned int iyear = EPOCH_YR; iyear < year; iyear++) {