1diff --git a/README.md b/README.md
  2index 4164f58..70516dc 100644
  3--- a/README.md
  4+++ b/README.md
  5@@ -48,6 +48,7 @@ Right now we're just using the number of days since 1900, but we should check th
  6 
  7 ### Numbers with commas in them should still parse to numbers.
  8 
  9+### ArgsChecker.checkLength should be typed
 10 
 11 ## Testing Guidelines
 12 
 13diff --git a/src/RawFormulas/Date.ts b/src/RawFormulas/Date.ts
 14index a9cab76..52f836e 100644
 15--- a/src/RawFormulas/Date.ts
 16+++ b/src/RawFormulas/Date.ts
 17@@ -24,9 +24,9 @@ import {
 18  */
 19 var DATE = function (...values) : ExcelDate {
 20   const FIRST_YEAR = 1900;
 21-  ArgsChecker.checkLength(values, 3);
 22   var year = Math.abs(Math.floor(TypeCaster.firstValueAsNumber(values[0]))); // No negative values for year
 23   var month = Math.floor(TypeCaster.firstValueAsNumber(values[1])) - 1; // Months are between 0 and 11.
 24+  ArgsChecker.checkLength(values, 3);
 25   var day = Math.floor(TypeCaster.firstValueAsNumber(values[2])) - 1; // Days are also zero-indexed.
 26   var m = moment.utc(ORIGIN_MOMENT)
 27     .add(2, "days")
 28@@ -767,8 +767,14 @@ var NETWORKDAYS$INTL = function (...values) : number {
 29   return networkDays;
 30 };
 31 
 32-var NOW = function () {
 33-  ArgsChecker.checkLength(arguments.length, 0);
 34+/**
 35+ * Returns the current date and time as a date value.
 36+ * @returns {ExcelDate} representing the current date and time.
 37+ * @constructor
 38+ */
 39+var NOW = function (...values) : ExcelDate {
 40+  ArgsChecker.checkLength(values, 0);
 41+  return new ExcelDate(moment());
 42 };
 43 
 44 
 45@@ -797,5 +803,6 @@ export {
 46   MINUTE,
 47   SECOND,
 48   NETWORKDAYS,
 49-  NETWORKDAYS$INTL
 50+  NETWORKDAYS$INTL,
 51+  NOW
 52 }
 53\ No newline at end of file
 54diff --git a/src/RawFormulas/RawFormulas.ts b/src/RawFormulas/RawFormulas.ts
 55index 71e151e..0da8087 100644
 56--- a/src/RawFormulas/RawFormulas.ts
 57+++ b/src/RawFormulas/RawFormulas.ts
 58@@ -127,7 +127,8 @@ import {
 59   MINUTE,
 60   SECOND,
 61   NETWORKDAYS,
 62-  NETWORKDAYS$INTL
 63+  NETWORKDAYS$INTL,
 64+  NOW
 65 } from "./Date"
 66 
 67 var ACCRINT = Formula["ACCRINT"];
 68@@ -258,5 +259,6 @@ export {
 69   MINUTE,
 70   SECOND,
 71   NETWORKDAYS,
 72-  NETWORKDAYS$INTL
 73+  NETWORKDAYS$INTL,
 74+  NOW
 75 }
 76\ No newline at end of file
 77diff --git a/tests/DateFormulasTest.ts b/tests/DateFormulasTest.ts
 78index bcc8ad7..29116ef 100644
 79--- a/tests/DateFormulasTest.ts
 80+++ b/tests/DateFormulasTest.ts
 81@@ -21,25 +21,12 @@ import {
 82   NETWORKDAYS$INTL
 83 } from "../src/RawFormulas/RawFormulas"
 84 import * as ERRORS from "../src/Errors"
 85-import {assertEquals} from "./utils/Asserts"
 86+import {
 87+  assertEquals,
 88+  catchAndAssertEquals
 89+} from "./utils/Asserts"
 90 import moment = require("moment");
 91 
 92-function catchAndAssertEquals(toExecute, expected) {
 93-  var toThrow = null;
 94-  try {
 95-    toExecute();
 96-    toThrow = true;
 97-  } catch (actualError) {
 98-    if (actualError.name !== expected) {
 99-      console.log("expected:", expected, " actual:", actualError.name);
100-      console.trace();
101-    }
102-  }
103-  if (toThrow) {
104-    console.log("expected error: " + expected);
105-    console.trace();
106-  }
107-}
108 
109 // Test NETWORKDAYS$INTL
110 assertEquals(NETWORKDAYS$INTL("1992-1-1", "1992-1-30"), 22);
111diff --git a/tests/DateFormulasTestTimeOverride.ts b/tests/DateFormulasTestTimeOverride.ts
112new file mode 100644
113index 0000000..960f28a
114--- /dev/null
115+++ b/tests/DateFormulasTestTimeOverride.ts
116@@ -0,0 +1,35 @@
117+
118+import {
119+  NOW,
120+  DATEVALUE
121+} from "../src/RawFormulas/RawFormulas"
122+import * as ERRORS from "../src/Errors"
123+import {
124+  assertEquals,
125+  catchAndAssertEquals
126+} from "./utils/Asserts"
127+
128+
129+// WARNING: Locking in Date by overriding prototypes.
130+function lockDate(year, month, day, hour, minute, second) {
131+  // WARNING: Locking in Date by overriding prototypes.
132+  var d = new Date(year, month, day, hour, minute, second); // Always "Dec 10 2012 04:55:04"
133+  Date.prototype.constructor = function () {
134+    return d;
135+  };
136+  Date.now = function () {
137+    return +(d);
138+  };
139+}
140+
141+lockDate(2012, 11, 10, 4, 55, 4);
142+assertEquals(NOW().toNumber(), DATEVALUE("Dec 10 2012"));
143+lockDate(1999, 11, 10, 4, 55, 4);
144+assertEquals(NOW().toNumber(), DATEVALUE("Dec 10 1999"));
145+lockDate(1999, 9, 22, 4, 55, 4);
146+assertEquals(NOW().toNumber(), DATEVALUE("Oct 22 1999"));
147+lockDate(1944, 1, 2, 1, 11, 55);
148+assertEquals(NOW().toNumber(), DATEVALUE("Feb 2 1944"));
149+catchAndAssertEquals(function() {
150+  NOW(12);
151+}, ERRORS.NA_ERROR);
152\ No newline at end of file
153diff --git a/tests/utils/Asserts.ts b/tests/utils/Asserts.ts
154index 49560d5..2851607 100644
155--- a/tests/utils/Asserts.ts
156+++ b/tests/utils/Asserts.ts
157@@ -38,7 +38,30 @@ function assertArrayEquals(actual: Array<any>, expected: Array<any>, ) {
158   }
159 }
160 
161+/**
162+ * Catch exceptions, check their name for an expected value
163+ * @param toExecute function to execute
164+ * @param expected error message
165+ */
166+function catchAndAssertEquals(toExecute : Function, expected) {
167+  var toThrow = null;
168+  try {
169+    toExecute();
170+    toThrow = true;
171+  } catch (actualError) {
172+    if (actualError.name !== expected) {
173+      console.log("expected:", expected, " actual:", actualError.name);
174+      console.trace();
175+    }
176+  }
177+  if (toThrow) {
178+    console.log("expected error: " + expected);
179+    console.trace();
180+  }
181+}
182+
183 export {
184   assertEquals,
185-  assertArrayEquals
186+  assertArrayEquals,
187+  catchAndAssertEquals
188 }
189\ No newline at end of file