1diff --git a/README.md b/README.md
  2index 609e2e9..ba89d27 100644
  3--- a/README.md
  4+++ b/README.md
  5@@ -10,8 +10,8 @@ And the same for MAX, MAXA, COUNT, COUNTA, etc. Look these over.
  6 ### Criteria evaluations should escape reg-ex characters
  7 http://stackoverflow.com/questions/3446170/escape-string-for-use-in-javascript-regex
  8 
  9-### jStat functions should know their caller
 10-Either through `arguments`, or directly passed in like `mean("FORMULA", [10, 20, 30])`
 11+### functions that throw errors should usually be able to include their caller in the error
 12+e.g. "SUM expects number values...", or "This function expects number values..."
 13 
 14 ### Dollar functions have special types
 15 Although dollar functions look like they just format `number`s, it seems like they have a special type under the hood.
 16@@ -39,18 +39,10 @@ Right now we're just using the number of days since 1900, but we should check th
 17 
 18 ### Verify that all N-times ({2,9}) are correct, and we're not parsing numbers too big.
 19 
 20-### Scrape jsdocs from functions, put in simple index.html, doc.md files to serve up simple documentation
 21+### Scrape jsdocs for functions, put in simple index.html, doc.md files to serve up simple documentation
 22 
 23 ### Numbers with commas in them should still parse to numbers.
 24 
 25 ### Ensure all formulas are tested inside of SheetFormulaTest.ts
 26 
 27-
 28-## Testing Guidelines
 29-
 30-All formulas should test for:
 31-1) One *less* argument than the formula expects, and one *more* argument than the formula expects.
 32-2) If it accepts a number, test with false as 0, and true as 1.
 33-3) If it accepts a number, test with string parsing to number.
 34-4) If it accepts a date, test with number, number as string, date as string.
 35-5) Individual arguments as ranges with single values (eg: `[1]`), and ranges as multiple values (eg: `[1, "str"]`).
 36+### Test all functions in src/Utilities
 37diff --git a/src/Formulas/Date.ts b/src/Formulas/Date.ts
 38index 684fa43..b37cd9b 100644
 39--- a/src/Formulas/Date.ts
 40+++ b/src/Formulas/Date.ts
 41@@ -263,6 +263,23 @@ var WEEKDAY = function (...values) {
 42 };
 43 
 44 
 45+/**
 46+ * Given a moment, an array of days of the week for shifting, will calculate the week number.
 47+ * @param dm moment to iterate towards
 48+ * @param shifterArray array of numbers for mapping week days to shifted weekdays
 49+ * @returns {number} of weeks in year
 50+ */
 51+function calculateWeekNum(dm : moment.Moment, shifterArray : Array<number>) : number {
 52+  var startOfYear = moment.utc(dm).startOf("year");
 53+  var weeksCount = 1;
 54+  var d = moment.utc(dm).startOf("year").add(6 - shifterArray[startOfYear.day()], "days");
 55+  while (d.isBefore(dm)) {
 56+    d.add(7, "days");
 57+    weeksCount++;
 58+  }
 59+  return weeksCount;
 60+}
 61+
 62 /**
 63  * Returns a number representing the week of the year where the provided date falls. When inputting the date, it is best
 64  * to use the DATE function, as text values may return errors.
 65@@ -278,7 +295,6 @@ var WEEKDAY = function (...values) {
 66  * the system used for determining the first week of the year (1=Sunday, 2=Monday).
 67  * @returns {number} representing week number of year.
 68  * @constructor
 69- * TODO: The performance of this function could be improved.
 70  */
 71 var WEEKNUM = function (...values) {
 72   ArgsChecker.checkLengthWithin(values, 1, 2);
 73@@ -322,44 +338,13 @@ var WEEKNUM = function (...values) {
 74     }
 75     return week;
 76   } else if (shiftType === 14) {
 77-    var SHIFTER = [3, 4, 5, 6, 0, 1, 2];
 78-    var startOfYear = moment.utc(dm).startOf("year");
 79-    var weeksCount = 1;
 80-    var d = moment.utc(dm).startOf("year").add(6 - SHIFTER[startOfYear.day()], "days");
 81-    while (d.isBefore(dm)) {
 82-      d.add(7, "days");
 83-      weeksCount++;
 84-    }
 85-    return weeksCount;
 86+    return calculateWeekNum(dm, [3, 4, 5, 6, 0, 1, 2]);
 87   } else if (shiftType === 15) {
 88-    var SHIFTER = [2, 3, 4, 5, 6, 0, 1];
 89-    var startOfYear = moment.utc(dm).startOf("year");
 90-    var weeksCount = 1;
 91-    var d = moment.utc(dm).startOf("year").add(6 - SHIFTER[startOfYear.day()], "days");
 92-    while (d.isBefore(dm)) {
 93-      d.add(7, "days");
 94-      weeksCount++;
 95-    }
 96-    return weeksCount;
 97+    return calculateWeekNum(dm, [2, 3, 4, 5, 6, 0, 1]);
 98   } else if (shiftType === 16) {
 99-    var SHIFTER = [1, 2, 3, 4, 5, 6, 0];
100-    var startOfYear = moment.utc(dm).startOf("year");
101-    var weeksCount = 1;
102-    var d = moment.utc(dm).startOf("year").add(6 - SHIFTER[startOfYear.day()], "days");
103-    while (d.isBefore(dm)) {
104-      d.add(7, "days");
105-      weeksCount++;
106-    }
107-    return weeksCount;
108+    return calculateWeekNum(dm, [1, 2, 3, 4, 5, 6, 0]);
109   } else if (shiftType === 17) {
110-    var startOfYear = moment.utc(dm).startOf("year");
111-    var weeksCount = 1;
112-    var d = moment.utc(dm).startOf("year").add(6 - startOfYear.day(), "days");
113-    while (d.isBefore(dm)) {
114-      d.add(7, "days");
115-      weeksCount++;
116-    }
117-    return weeksCount;
118+    return calculateWeekNum(dm, [0, 1, 2, 3, 4, 5, 6]);
119   } else if (shiftType === 21) {
120     return dm.isoWeek();
121   } else {
122@@ -420,10 +405,6 @@ var DATEDIF = function (...values) : number {
123     var months = Math.floor(e.diff(s, "months"));
124     return months === 12 ? 0 : months;
125   } else if (unitClean === "YD") {
126-    // var s = start.toMoment();
127-    // var e = end.toMoment();
128-    // var days = e.diff(s, "days");
129-    // return days
130     var s = start.toMoment();
131     var e = end.toMoment();
132     while(s.isBefore(e)) {