diff --git a/src/main/java/org/javarosa/xpath/expr/XPathFuncExpr.java b/src/main/java/org/javarosa/xpath/expr/XPathFuncExpr.java
index 6699cea54..cc7d770fa 100644
--- a/src/main/java/org/javarosa/xpath/expr/XPathFuncExpr.java
+++ b/src/main/java/org/javarosa/xpath/expr/XPathFuncExpr.java
@@ -699,7 +699,7 @@ public static Double toNumeric(Object o) {
                 val = NaN;
             }
         } else if (o instanceof Date) {
-            val = (double) DateUtils.daysSinceEpoch((Date) o);
+            val = (double) DateUtils.fractionalDaysSinceEpoch((Date) o);
         } else if (o instanceof IExprDataType) {
             val = ((IExprDataType) o).toNumeric();
         }
@@ -759,7 +759,7 @@ public static String toString(Object o) {
         } else if (o instanceof String) {
             val = (String) o;
         } else if (o instanceof Date) {
-            val = DateUtils.formatDate((Date) o, DateUtils.FORMAT_ISO8601);
+            val = DateUtils.formatDateTime((Date) o, DateUtils.FORMAT_ISO8601);
         } else if (o instanceof IExprDataType) {
             val = ((IExprDataType) o).toString();
         }
diff --git a/src/test/java/org/javarosa/xpath/test/XPathEvalTest.java b/src/test/java/org/javarosa/xpath/test/XPathEvalTest.java
index c354511e2..a11170b42 100644
--- a/src/test/java/org/javarosa/xpath/test/XPathEvalTest.java
+++ b/src/test/java/org/javarosa/xpath/test/XPathEvalTest.java
@@ -198,7 +198,8 @@ public void type_conversions() {
         testEval("string('')", "");
         testEval("string('  ')", "  ");
         testEval("string('a string')", "a string");
-        testEval("string(date('1989-11-09'))", "1989-11-09");
+        //For #637
+        testEval("string(date('1989-11-09'))", "1989-11-09T00:00:00.000Z");
         testEval("string(convertible())", null, ec, "hi");
         testEval("string(inconvertible())", null, ec, new XPathTypeMismatchException());
         testEval("int('100')", 100.0);