1diff --git a/src/RawFormulas/Date.ts b/src/RawFormulas/Date.ts
  2index 528f509..8ec43db 100644
  3--- a/src/RawFormulas/Date.ts
  4+++ b/src/RawFormulas/Date.ts
  5@@ -257,20 +257,82 @@ var WEEKDAY = function (...values) {
  6 };
  7 
  8 
  9+/**
 10+ * Returns a number representing the week of the year where the provided date falls. When inputting the date, it is best
 11+ * to use the DATE function, as text values may return errors.
 12+ *
 13+ * Behind the scenes, there are two week numbering "systems" used for this function: System 1 - The first week of the
 14+ * year is considered to be the week containing January 1, which is numbered week 1. System 2 - The first week of the
 15+ * year is considered to be the week containing the first Thursday of the year, which is numbered as week 1. System 2 is
 16+ * the approach specified in ISO 8601, also known as the European system for numbering weeks.
 17+ *
 18+ * @param values[0] date - The date for which to determine the week number. Must be a reference to a cell containing a
 19+ * date, a function returning a date type, or a number.
 20+ * @param values[1] type - [ OPTIONAL - default is 1 ] - A number representing the day that a week starts on as well as
 21+ * the system used for determining the first week of the year (1=Sunday, 2=Monday).
 22+ * @returns {number} representing week number of year.
 23+ * @constructor
 24+ */
 25 var WEEKNUM = function (...values) {
 26-  ArgsChecker.checkLength(values, 1);
 27+  ArgsChecker.checkLengthWithin(values, 1, 2);
 28   var date = TypeCaster.firstValueAsExcelDate(values[0], true); // tell firstValueAsExcelDate to coerce boolean
 29+  var shiftType = values.length === 2 ? TypeCaster.firstValueAsNumber(values[1]) : 1;
 30   if (date.toNumber() < 0) {
 31     throw new NumError("Function YEAR parameter 1 value is " + date.toNumber() + ". It should be greater than or equal to 0.");
 32   }
 33   var dm = date.toMoment();
 34   var week = dm.week();
 35-  // If this weekYear is not the same as the year, then we're technically in "week-53"
 36-  // See https://momentjs.com/docs/#/get-set/week-year/ for more info.
 37-  if (dm.weekYear() !== dm.year()) {
 38-    return 53;
 39+  var dayOfWeek = dm.day() + 1; // between 1 and 7, inclusively
 40+  if (shiftType === 1) {
 41+    // If this weekYear is not the same as the year, then we're technically in "week 53"
 42+    // See https://momentjs.com/docs/#/get-set/week-year/ for more info.
 43+    if (dm.weekYear() !== dm.year()) {
 44+      week = 53;
 45+    }
 46+    return week;
 47+  } else if (shiftType === 2 || shiftType === 11) {
 48+    if (dm.weekYear() !== dm.year()) {
 49+      week = 53;
 50+    }
 51+    if (dayOfWeek === 1) { // sunday shift back
 52+      return week - 1;
 53+    }
 54+    return week;
 55+  } else if (shiftType === 12) {
 56+    if (dm.weekYear() !== dm.year()) {
 57+      week = 53;
 58+    }
 59+    if (dayOfWeek <= 2) { // sunday, monday shift back
 60+      return week - 1;
 61+    }
 62+    return week;
 63+  } else if (shiftType === 13) {
 64+    if (dm.weekYear() !== dm.year()) {
 65+      week = 53;
 66+    }
 67+    if (dayOfWeek <= 3) { // sunday, monday, tuesday shift back
 68+      return week - 1;
 69+    }
 70+    return week;
 71+  } else if (shiftType === 14) {
 72+    if (dm.weekYear() !== dm.year()) {
 73+      week = 53;
 74+    }
 75+    if (dayOfWeek <= 4) { // sunday, monday, tuesday, wednesday shift back
 76+      return week - 1;
 77+    }
 78+    return week;
 79+  } else if (shiftType === 15) {
 80+
 81+  } else if (shiftType === 16) {
 82+
 83+  } else if (shiftType === 17) {
 84+
 85+  } else if (shiftType === 21) {
 86+
 87+  } else {
 88+    throw new NumError("Function WEEKNUM parameter 2 value " + shiftType + " is out of range.");
 89   }
 90-  return week;
 91 };
 92 
 93 
 94diff --git a/tests/DateFormulasTest.ts b/tests/DateFormulasTest.ts
 95index 4e59172..78f7e03 100644
 96--- a/tests/DateFormulasTest.ts
 97+++ b/tests/DateFormulasTest.ts
 98@@ -76,9 +76,134 @@ assertEquals(WEEKNUM(23749), 2);
 99 assertEquals(WEEKNUM(23750), 2);
100 assertEquals(WEEKNUM(23751), 2);
101 assertEquals(WEEKNUM(23752), 3);
102-
103-
104-
105+// type=2
106+assertEquals(WEEKNUM(23737, 2), 52);
107+assertEquals(WEEKNUM(23738, 2), 52);
108+assertEquals(WEEKNUM(23739, 2), 53);
109+assertEquals(WEEKNUM(23740, 2), 53);
110+assertEquals(WEEKNUM(23741, 2), 53);
111+assertEquals(WEEKNUM(23742, 2), 53);
112+assertEquals(WEEKNUM(23743, 2), 1);
113+assertEquals(WEEKNUM(23744, 2), 1);
114+assertEquals(WEEKNUM(23745, 2), 1);
115+assertEquals(WEEKNUM(23746, 2), 2);
116+assertEquals(WEEKNUM(23747, 2), 2);
117+assertEquals(WEEKNUM(23748, 2), 2);
118+assertEquals(WEEKNUM(23749, 2), 2);
119+assertEquals(WEEKNUM(23750, 2), 2);
120+assertEquals(WEEKNUM(23751, 2), 2);
121+assertEquals(WEEKNUM(23752, 2), 2);
122+assertEquals(WEEKNUM(23753, 2), 3);
123+assertEquals(WEEKNUM(23754, 2), 3);
124+assertEquals(WEEKNUM(23755, 2), 3);
125+assertEquals(WEEKNUM(23756, 2), 3);
126+assertEquals(WEEKNUM(23757, 2), 3);
127+assertEquals(WEEKNUM(23758, 2), 3);
128+assertEquals(WEEKNUM(23759, 2), 3);
129+assertEquals(WEEKNUM(23760, 2), 4);
130+//type=11
131+assertEquals(WEEKNUM(23737, 11), 52);
132+assertEquals(WEEKNUM(23738, 11), 52);
133+assertEquals(WEEKNUM(23739, 11), 53);
134+assertEquals(WEEKNUM(23740, 11), 53);
135+assertEquals(WEEKNUM(23741, 11), 53);
136+assertEquals(WEEKNUM(23742, 11), 53);
137+assertEquals(WEEKNUM(23743, 11), 1);
138+assertEquals(WEEKNUM(23744, 11), 1);
139+assertEquals(WEEKNUM(23745, 11), 1);
140+assertEquals(WEEKNUM(23746, 11), 2);
141+assertEquals(WEEKNUM(23747, 11), 2);
142+assertEquals(WEEKNUM(23748, 11), 2);
143+assertEquals(WEEKNUM(23749, 11), 2);
144+assertEquals(WEEKNUM(23750, 11), 2);
145+assertEquals(WEEKNUM(23751, 11), 2);
146+assertEquals(WEEKNUM(23752, 11), 2);
147+assertEquals(WEEKNUM(23753, 11), 3);
148+assertEquals(WEEKNUM(23754, 11), 3);
149+assertEquals(WEEKNUM(23755, 11), 3);
150+assertEquals(WEEKNUM(23756, 11), 3);
151+assertEquals(WEEKNUM(23757, 11), 3);
152+assertEquals(WEEKNUM(23758, 11), 3);
153+assertEquals(WEEKNUM(23759, 11), 3);
154+assertEquals(WEEKNUM(23760, 11), 4);
155+//type=12
156+assertEquals(WEEKNUM(23737, 12), 52);
157+assertEquals(WEEKNUM(23738, 12), 52);
158+assertEquals(WEEKNUM(23739, 12), 52);
159+assertEquals(WEEKNUM(23740, 12), 53);
160+assertEquals(WEEKNUM(23741, 12), 53);
161+assertEquals(WEEKNUM(23742, 12), 53);
162+assertEquals(WEEKNUM(23743, 12), 1);
163+assertEquals(WEEKNUM(23744, 12), 1);
164+assertEquals(WEEKNUM(23745, 12), 1);
165+assertEquals(WEEKNUM(23746, 12), 1);
166+assertEquals(WEEKNUM(23747, 12), 2);
167+assertEquals(WEEKNUM(23748, 12), 2);
168+assertEquals(WEEKNUM(23749, 12), 2);
169+assertEquals(WEEKNUM(23750, 12), 2);
170+assertEquals(WEEKNUM(23751, 12), 2);
171+assertEquals(WEEKNUM(23752, 12), 2);
172+assertEquals(WEEKNUM(23753, 12), 2);
173+assertEquals(WEEKNUM(23754, 12), 3);
174+assertEquals(WEEKNUM(23755, 12), 3);
175+assertEquals(WEEKNUM(23756, 12), 3);
176+assertEquals(WEEKNUM(23757, 12), 3);
177+assertEquals(WEEKNUM(23758, 12), 3);
178+assertEquals(WEEKNUM(23759, 12), 3);
179+assertEquals(WEEKNUM(23760, 12), 3);
180+//type=13
181+assertEquals(WEEKNUM(23737, 13), 52);
182+assertEquals(WEEKNUM(23738, 13), 52);
183+assertEquals(WEEKNUM(23739, 13), 52);
184+assertEquals(WEEKNUM(23740, 13), 52);
185+assertEquals(WEEKNUM(23741, 13), 53);
186+assertEquals(WEEKNUM(23742, 13), 53);
187+assertEquals(WEEKNUM(23743, 13), 1);
188+assertEquals(WEEKNUM(23744, 13), 1);
189+assertEquals(WEEKNUM(23745, 13), 1);
190+assertEquals(WEEKNUM(23746, 13), 1);
191+assertEquals(WEEKNUM(23747, 13), 1);
192+assertEquals(WEEKNUM(23748, 13), 2);
193+assertEquals(WEEKNUM(23749, 13), 2);
194+assertEquals(WEEKNUM(23750, 13), 2);
195+assertEquals(WEEKNUM(23751, 13), 2);
196+assertEquals(WEEKNUM(23752, 13), 2);
197+assertEquals(WEEKNUM(23753, 13), 2);
198+assertEquals(WEEKNUM(23754, 13), 2);
199+assertEquals(WEEKNUM(23755, 13), 3);
200+assertEquals(WEEKNUM(23756, 13), 3);
201+assertEquals(WEEKNUM(23757, 13), 3);
202+assertEquals(WEEKNUM(23758, 13), 3);
203+assertEquals(WEEKNUM(23759, 13), 3);
204+assertEquals(WEEKNUM(23760, 13), 3);
205+//type=14
206+assertEquals(WEEKNUM(23734, 14), 52);
207+assertEquals(WEEKNUM(23735, 14), 53);
208+assertEquals(WEEKNUM(23736, 14), 53);
209+assertEquals(WEEKNUM(23737, 14), 53);
210+assertEquals(WEEKNUM(23738, 14), 53);
211+assertEquals(WEEKNUM(23739, 14), 53);
212+assertEquals(WEEKNUM(23740, 14), 53);
213+assertEquals(WEEKNUM(23741, 14), 53);
214+assertEquals(WEEKNUM(23742, 14), 54);
215+assertEquals(WEEKNUM(23743, 14), 1);
216+assertEquals(WEEKNUM(23744, 14), 1);
217+assertEquals(WEEKNUM(23745, 14), 1);
218+assertEquals(WEEKNUM(23746, 14), 1);
219+assertEquals(WEEKNUM(23747, 14), 1);
220+assertEquals(WEEKNUM(23748, 14), 1);
221+assertEquals(WEEKNUM(23749, 14), 2);
222+assertEquals(WEEKNUM(23750, 14), 2);
223+assertEquals(WEEKNUM(23751, 14), 2);
224+assertEquals(WEEKNUM(23752, 14), 2);
225+assertEquals(WEEKNUM(23753, 14), 2);
226+assertEquals(WEEKNUM(23754, 14), 2);
227+assertEquals(WEEKNUM(23755, 14), 2);
228+assertEquals(WEEKNUM(23756, 14), 3);
229+assertEquals(WEEKNUM(23757, 14), 3);
230+assertEquals(WEEKNUM(23758, 14), 3);
231+assertEquals(WEEKNUM(23759, 14), 3);
232+assertEquals(WEEKNUM(23760, 14), 3);
233 
234 
235 // Test WEEKDAY