diff --git a/__tests__/__main__/validate-json.js b/__tests__/__main__/validate-json.js
index 9e0356a4..d7351dcb 100644
--- a/__tests__/__main__/validate-json.js
+++ b/__tests__/__main__/validate-json.js
@@ -32,10 +32,9 @@ describe('Validate json', function()
             const goodFlexibleEntry = [{ 'type': 'flexible', 'date': '2020-06-03', 'values': ['08:00', '12:00', '13:00', '14:00'] }];
             const goodWaivedEntry = [{ 'type': 'waived', 'date': '2020-06-03', 'data': 'waived', 'hours': '08:00' }];
             const invalidDateFormat = [{ 'type': 'flexible', 'date': '03-06-2020', 'values': ['08:00', '12:00', '13:00', '14:00'] }];
-            const invalidDateType = [{ 'type': 'flexible', 'date': ['2020-06-03'], 'values': ['08:00', '12:00', '13:00', '14:00'] }];
+            const invalidDateType = [{ 'type': 'flexible', 'date': ['2020-06-13'], 'values': ['08:00', '12:00', '13:00', '14:00'] }];
             const invalidDateValue = [{ 'type': 'flexible', 'date': '2020-26-03', 'values': ['08:00', '12:00', '13:00', '14:00'] }];
-            // TODO
-            //const invalidDayInMonth = [{ 'type': 'flexible', 'date': '2020-02-31', 'values': ['08:00', '12:00', '13:00', '14:00'] }];
+            const invalidDayInMonth = [{ 'type': 'flexible', 'date': '2020-04-31', 'values': ['08:00', '12:00', '13:00', '14:00'] }];
             test('should be valid JSON', () =>
             {
                 expect(validateJSON(goodWaivedEntry)).toBeTruthy();
@@ -46,7 +45,7 @@ describe('Validate json', function()
                 expect(validateJSON(invalidDateFormat)).not.toBeTruthy();
                 expect(validateJSON(invalidDateType)).not.toBeTruthy();
                 expect(validateJSON(invalidDateValue)).not.toBeTruthy();
-                //expect(validateJSON(invalidDayInMonth)).not.toBeTruthy();
+                expect(validateJSON(invalidDayInMonth)).not.toBeTruthy();
             });
         });
 
diff --git a/js/validate-json.js b/js/validate-json.js
index 018afd57..65ba8469 100644
--- a/js/validate-json.js
+++ b/js/validate-json.js
@@ -22,7 +22,8 @@ const schemaWaivedEntry = {
         ,
         'date': {
             'type': 'string',
-            'pattern': /(1|2)[0-9]{3}-(0[1-9]{1}|1[0-1]{1})-(0[0-9]{1}|1[0-9]{1}|2[0-9]{1}|3[0-1]{1})$/
+            'pattern': /(1|2)[0-9]{3}-(0[1-9]{1}|1[0-1]{1})-(0[0-9]{1}|1[0-9]{1}|2[0-9]{1}|3[0-1]{1})$/,
+            'format': 'dateFormat'
         },
         'data': {
             'type': 'string'
@@ -49,7 +50,8 @@ const schemaFlexibleEntry = {
         ,
         'date': {
             'type': 'string',
-            'pattern': /(1|2)[0-9]{3}-(0[1-9]{1}|1[0-1]{1})-(0[0-9]{1}|1[0-9]{1}|2[0-9]{1}|3[0-1]{1})$/
+            'pattern': /(1|2)[0-9]{3}-(0[1-9]{1}|1[0-1]{1})-(0[0-9]{1}|1[0-9]{1}|2[0-9]{1}|3[0-1]{1})$/,
+            'format': 'dateFormat'
         },
         'values': {
             'type': 'array',
@@ -66,6 +68,35 @@ const schemaFlexibleEntry = {
     ]
 };
 
+function daysInMonth(m, y)
+{
+    switch (m)
+    {
+    case 1 :
+        return (y % 4 === 0 && y % 100) || y % 400 === 0 ? 29 : 28;
+    case 8 : case 3 : case 5 : case 10 :
+        return 30;
+    default :
+        return 31;
+    }
+}
+
+function isValid(y, m, d)
+{
+    return m >= 0 && m < 12 && d > 0 && d <= daysInMonth(m, y);
+}
+
+Validator.prototype.customFormats.dateFormat = function(dateStr)
+{
+    if (!typeof(dateStr) === 'String' || !dateStr.includes('-'))
+    {
+        return false;
+    }
+    const dateArray = dateStr.split('-');
+    const date = new Date(dateArray[0],dateArray[1], dateArray[2]);
+    const validDate = (date instanceof(Date) && isFinite(date.getTime()) && isValid(dateArray[0],dateArray[1]-1, dateArray[2]));
+    return validDate;
+};
 
 function validateJSON(instance)
 {