Index: src/of_strptime.m
==================================================================
--- src/of_strptime.m
+++ src/of_strptime.m
@@ -112,33 +112,33 @@
 			case 'b':
 				if (bufferLen < j + 3)
 					return NULL;
 
 				if (memcmp(buffer + j, "Jan", 3) == 0)
-					tm->tm_mon = 1;
+					tm->tm_mon = 0;
 				else if (memcmp(buffer + j, "Feb", 3) == 0)
-					tm->tm_mon = 2;
+					tm->tm_mon = 1;
 				else if (memcmp(buffer + j, "Mar", 3) == 0)
-					tm->tm_mon = 3;
+					tm->tm_mon = 2;
 				else if (memcmp(buffer + j, "Apr", 3) == 0)
-					tm->tm_mon = 4;
+					tm->tm_mon = 3;
 				else if (memcmp(buffer + j, "May", 3) == 0)
-					tm->tm_mon = 5;
+					tm->tm_mon = 4;
 				else if (memcmp(buffer + j, "Jun", 3) == 0)
-					tm->tm_mon = 6;
+					tm->tm_mon = 5;
 				else if (memcmp(buffer + j, "Jul", 3) == 0)
-					tm->tm_mon = 7;
+					tm->tm_mon = 6;
 				else if (memcmp(buffer + j, "Aug", 3) == 0)
-					tm->tm_mon = 8;
+					tm->tm_mon = 7;
 				else if (memcmp(buffer + j, "Sep", 3) == 0)
-					tm->tm_mon = 9;
+					tm->tm_mon = 8;
 				else if (memcmp(buffer + j, "Oct", 3) == 0)
-					tm->tm_mon = 10;
+					tm->tm_mon = 9;
 				else if (memcmp(buffer + j, "Nov", 3) == 0)
-					tm->tm_mon = 11;
+					tm->tm_mon = 10;
 				else if (memcmp(buffer + j, "Dec", 3) == 0)
-					tm->tm_mon = 12;
+					tm->tm_mon = 11;
 				else
 					return NULL;
 
 				j += 3;
 				break;

Index: tests/OFDateTests.m
==================================================================
--- tests/OFDateTests.m
+++ tests/OFDateTests.m
@@ -39,11 +39,11 @@
 	struct tm tm;
 	int16_t tz;
 	const char *dstr = "Wed, 09 Jun 2021 +0200x";
 	TEST(@"of_strptime()",
 	    of_strptime(dstr, "%a, %d %b %Y %z", &tm, &tz) == dstr + 22 &&
-	    tm.tm_wday == 3 && tm.tm_mday == 9 && tm.tm_mon == 6 &&
+	    tm.tm_wday == 3 && tm.tm_mday == 9 && tm.tm_mon == 5 &&
 	    tm.tm_year == 2021 - 1900 && tz == 2 * 60)
 
 	TEST(@"+[dateWithTimeIntervalSince1970:]",
 	    (d1 = [OFDate dateWithTimeIntervalSince1970: 0]))