From f2602d57c504e30f4daa3c0518c9f296da9f6caa Mon Sep 17 00:00:00 2001
From: Steve Broshar <chillinwithscb@outlook.com>
Date: Fri, 28 Oct 2016 16:41:07 -0500
Subject: [PATCH] Handle DATE() parameters the same as excel does.

---
 lib/date-time.js  |  4 +++-
 test/date-time.js | 42 ++++++++++++++++++++++++++++++++++--------
 2 files changed, 37 insertions(+), 9 deletions(-)

diff --git a/lib/date-time.js b/lib/date-time.js
index 9234643f..f69a4f79 100644
--- a/lib/date-time.js
+++ b/lib/date-time.js
@@ -70,9 +70,11 @@ exports.DATE = function(year, month, day) {
   if (utils.anyIsError(year, month, day)) {
     return error.value;
   }
-  if (year < 0 || month < 0 || day < 0) {
+  if (year < 0 || year > 9999) {
     return error.num;
   }
+  if (year < 1900)
+    year += 1900;
   var date = new Date(year, month - 1, day);
   return date;
 };
diff --git a/test/date-time.js b/test/date-time.js
index 4856e7cf..e98308b7 100644
--- a/test/date-time.js
+++ b/test/date-time.js
@@ -4,14 +4,40 @@ var dateTime = require('../lib/date-time');
 require('should');
 
 suite('Date & Time', function() {
-  test('DATE', function() {
-    var date = dateTime.DATE(1900, 1, 1);
-    date.getFullYear().should.equal(1900);
-    date.getMonth().should.equal(1 - 1);
-    date.getDay().should.equal(1);
-
-    dateTime.DATE(1900, 1, -1).should.equal(error.num);
-    dateTime.DATE('invalid').should.equal(error.value);
+  suite('DATE', function (){
+      test('returns Date for year, month, day', function(){
+        var date = dateTime.DATE(1900, 1, 1);
+        date.getFullYear().should.equal(1900);
+        date.getMonth().should.equal(1 - 1);
+        date.getDay().should.equal(1);          
+      });
+      test('uses year as is for 1900 to 9999', function() {
+        dateTime.DATE(1900, 2, 2).getFullYear().should.equal(1900);
+        dateTime.DATE(9999, 2, 2).getFullYear().should.equal(9999);
+      });
+      test('adds 1900 to year for 0 to 1899', function() {
+        dateTime.DATE(0, 2, 2).getFullYear().should.equal(1900);
+        dateTime.DATE(1899, 2, 2).getFullYear().should.equal(3799);
+      });
+      test('returns num error for year <0 or >9999', function() {
+        dateTime.DATE(-1, 2, 2).should.equal(error.num);
+        dateTime.DATE(10000, 2, 2).should.equal(error.num);
+      });
+      test('adds months >12', function() {
+        dateTime.DATE(2008, 14, 2).should.eql(new Date(2009, 1, 2));
+      });
+      test('subtracts months <0', function() {
+        dateTime.DATE(2008, -3, 2).should.eql(new Date(2007, 8, 2));
+      });
+      test('adds days > month has', function() {
+        dateTime.DATE(2008, 1, 35).should.eql(new Date(2008, 1, 4));
+      });
+      test('subtracts days <0', function() {
+        dateTime.DATE(2008, 1, -15).should.eql(new Date(2007, 11, 16));
+      });
+      test('returns value error for string', function(){
+        dateTime.DATE('invalid').should.equal(error.value);
+      });
   });
 
   test('DATEVALUE', function() {