diff --git a/src/utility/datetimeUtils.cpp b/src/utility/datetimeUtils.cpp
index cf05a932..6ea5be60 100644
--- a/src/utility/datetimeUtils.cpp
+++ b/src/utility/datetimeUtils.cpp
@@ -256,7 +256,7 @@ int datetimeUtils::getDayOfWeek(const int year, const int month, const int day)
 }
 
 
-int datetimeUtils::getWeekOfYear(const int year, const int month, const int day)
+int datetimeUtils::getWeekOfYear(const int year, const int month, const int day, const bool iso)
 {
 	// Algorithm from http://personal.ecu.edu/mccartyr/ISOwdALG.txt
 
@@ -322,7 +322,7 @@ int datetimeUtils::getWeekOfYear(const int year, const int month, const int day)
 			WeekNumber -= 1;
 	}
 
-	if (WeekNumber == 1 && month == 12)
+	if (!iso && (WeekNumber == 1 && month == 12))
 		WeekNumber = 53;
 
 	return WeekNumber;
diff --git a/tests/utility/datetimeUtilsTest.cpp b/tests/utility/datetimeUtilsTest.cpp
index 4fd3e436..4671e75b 100644
--- a/tests/utility/datetimeUtilsTest.cpp
+++ b/tests/utility/datetimeUtilsTest.cpp
@@ -143,10 +143,11 @@ VMIME_TEST_SUITE_BEGIN
 	{
 		VASSERT_EQ("1.1", 52, datetimeUtils::getWeekOfYear(2003, 12, 27));
 		VASSERT_EQ("1.2", 52, datetimeUtils::getWeekOfYear(2003, 12, 28));
-		VASSERT_EQ("1.3",  1, datetimeUtils::getWeekOfYear(2003, 12, 29));
-		VASSERT_EQ("1.4",  1, datetimeUtils::getWeekOfYear(2004,  1,  4));
-		VASSERT_EQ("1.5",  2, datetimeUtils::getWeekOfYear(2004,  1,  5));
-		VASSERT_EQ("1.6",  2, datetimeUtils::getWeekOfYear(2004,  1, 11));
+		VASSERT_EQ("1.3",  1, datetimeUtils::getWeekOfYear(2003, 12, 29, true));
+		VASSERT_EQ("1.4", 53, datetimeUtils::getWeekOfYear(2003, 12, 29, false));
+		VASSERT_EQ("1.5",  1, datetimeUtils::getWeekOfYear(2004,  1,  4));
+		VASSERT_EQ("1.6",  2, datetimeUtils::getWeekOfYear(2004,  1,  5));
+		VASSERT_EQ("1.7",  2, datetimeUtils::getWeekOfYear(2004,  1, 11));
 
 		VASSERT_EQ("2.1", 52, datetimeUtils::getWeekOfYear(2004, 12, 26));
 		VASSERT_EQ("2.2", 53, datetimeUtils::getWeekOfYear(2004, 12, 27));
diff --git a/vmime/utility/datetimeUtils.hpp b/vmime/utility/datetimeUtils.hpp
index b56533cb..d5f89beb 100644
--- a/vmime/utility/datetimeUtils.hpp
+++ b/vmime/utility/datetimeUtils.hpp
@@ -84,9 +84,11 @@ public:
 	  * @param year year in 4-digit format
 	  * @param month month (1-12), January is 1, December is 12 (see datetime::Months enum)
 	  * @param day month day (1-31)
+	  * @param iso if TRUE, use ISO week-numbering year (default is to use calendar year).
+	  * For more information, read here: http://en.wikipedia.org/wiki/ISO_8601#Week_dates
 	  * @return the week number (1 is the first week of the year)
 	  */
-	static int getWeekOfYear(const int year, const int month, const int day);
+	static int getWeekOfYear(const int year, const int month, const int day, const bool iso = false);
 };