diff --git a/src/main/java/graphql/scalars/datetime/DateTimeScalar.java b/src/main/java/graphql/scalars/datetime/DateTimeScalar.java
index 20fd345..06b3aa3 100644
--- a/src/main/java/graphql/scalars/datetime/DateTimeScalar.java
+++ b/src/main/java/graphql/scalars/datetime/DateTimeScalar.java
@@ -13,10 +13,16 @@
 import java.time.OffsetDateTime;
 import java.time.ZonedDateTime;
 import java.time.format.DateTimeFormatter;
+import java.time.format.DateTimeFormatterBuilder;
 import java.time.format.DateTimeParseException;
 import java.util.function.Function;
 
 import static graphql.scalars.util.Kit.typeName;
+import static java.time.format.DateTimeFormatter.ISO_LOCAL_DATE;
+import static java.time.temporal.ChronoField.HOUR_OF_DAY;
+import static java.time.temporal.ChronoField.MINUTE_OF_HOUR;
+import static java.time.temporal.ChronoField.NANO_OF_SECOND;
+import static java.time.temporal.ChronoField.SECOND_OF_MINUTE;
 
 /**
  * Access this via {@link graphql.scalars.ExtendedScalars#DateTime}
@@ -27,6 +33,7 @@ public final class DateTimeScalar {
     public static final GraphQLScalarType INSTANCE;
 
     private DateTimeScalar() {}
+    private static final DateTimeFormatter customOutputFormatter = getCustomDateTimeFormatter();
 
     static {
         Coercing<OffsetDateTime, String> coercing = new Coercing<OffsetDateTime, String>() {
@@ -45,7 +52,7 @@ public String serialize(Object input) throws CoercingSerializeException {
                     );
                 }
                 try {
-                    return DateTimeFormatter.ISO_OFFSET_DATE_TIME.format(offsetDateTime);
+                    return customOutputFormatter.format(offsetDateTime);
                 } catch (DateTimeException e) {
                     throw new CoercingSerializeException(
                             "Unable to turn TemporalAccessor into OffsetDateTime because of : '" + e.getMessage() + "'."
@@ -102,4 +109,20 @@ private OffsetDateTime parseOffsetDateTime(String s, Function<String, RuntimeExc
                 .build();
     }
 
+    private static DateTimeFormatter getCustomDateTimeFormatter() {
+        return new DateTimeFormatterBuilder()
+                .parseCaseInsensitive()
+                .append(ISO_LOCAL_DATE)
+                .appendLiteral('T')
+                .appendValue(HOUR_OF_DAY, 2)
+                .appendLiteral(':')
+                .appendValue(MINUTE_OF_HOUR, 2)
+                .appendLiteral(':')
+                .appendValue(SECOND_OF_MINUTE, 2)
+                .appendFraction(NANO_OF_SECOND, 3, 3, true)
+                .appendOffset("+HH:MM", "Z")
+                .toFormatter();
+    }
+
+
 }
diff --git a/src/test/groovy/graphql/scalars/datetime/DateTimeScalarTest.groovy b/src/test/groovy/graphql/scalars/datetime/DateTimeScalarTest.groovy
index 8184cf7..699907a 100644
--- a/src/test/groovy/graphql/scalars/datetime/DateTimeScalarTest.groovy
+++ b/src/test/groovy/graphql/scalars/datetime/DateTimeScalarTest.groovy
@@ -41,11 +41,12 @@ class DateTimeScalarTest extends Specification {
         result.isEqualTo(expectedValue)
         where:
         input                           | expectedValue
-        "1985-04-12T23:20:50.52Z"       | mkStringValue("1985-04-12T23:20:50.52Z")
-        "1996-12-19T16:39:57-08:00"     | mkStringValue("1996-12-19T16:39:57-08:00")
-        "1937-01-01T12:00:27.87+00:20"  | mkStringValue("1937-01-01T12:00:27.87+00:20")
-        mkOffsetDT(year: 1980, hour: 3) | mkStringValue("1980-08-08T03:10:09+10:00")
-        mkZonedDT(year: 1980, hour: 3)  | mkStringValue("1980-08-08T03:10:09+10:00")
+        "1985-04-12T23:20:50.52Z"       | mkStringValue("1985-04-12T23:20:50.520Z")
+        "1996-12-19T16:39:57-08:00"     | mkStringValue("1996-12-19T16:39:57.000-08:00")
+        "1937-01-01T12:00:27.87+00:20"  | mkStringValue("1937-01-01T12:00:27.870+00:20")
+        "1937-01-01T12:00+00:20"        | mkStringValue("1937-01-01T12:00:00.000+00:20")
+        mkOffsetDT(year: 1980, hour: 3) | mkStringValue("1980-08-08T03:10:09.000+10:00")
+        mkZonedDT(year: 1980, hour: 3)  | mkStringValue("1980-08-08T03:10:09.000+10:00")
     }
 
     @Unroll
@@ -81,11 +82,11 @@ class DateTimeScalarTest extends Specification {
         result == expectedValue
         where:
         input                           | expectedValue
-        "1985-04-12T23:20:50.52Z"       | "1985-04-12T23:20:50.52Z"
-        "1996-12-19T16:39:57-08:00"     | "1996-12-19T16:39:57-08:00"
-        "1937-01-01T12:00:27.87+00:20"  | "1937-01-01T12:00:27.87+00:20"
-        mkOffsetDT(year: 1980, hour: 3) | "1980-08-08T03:10:09+10:00"
-        mkZonedDT(year: 1980, hour: 3)  | "1980-08-08T03:10:09+10:00"
+        "1985-04-12T23:20:50.52Z"       | "1985-04-12T23:20:50.520Z"
+        "1996-12-19T16:39:57-08:00"     | "1996-12-19T16:39:57.000-08:00"
+        "1937-01-01T12:00:27.87+00:20"  | "1937-01-01T12:00:27.870+00:20"
+        mkOffsetDT(year: 1980, hour: 3) | "1980-08-08T03:10:09.000+10:00"
+        mkZonedDT(year: 1980, hour: 3)  | "1980-08-08T03:10:09.000+10:00"
     }
 
     def "datetime serialisation bad inputs"() {