fix #3505 in the best way we can for 6.x

(cherry picked from commit b414ad9)

Author: Mpdreamz

src/Nest/CommonOptions/DateMath/DateMath.cs

@@ -79,10 +79,11 @@ public static DateMath FromString(string dateMath)
 			return math;
 		}
 
+		internal bool IsValid => Self.Anchor.Match(d => d != default, s => !s.IsNullOrEmpty());
+
 		public override string ToString()
 		{
-			var isValid = Self.Anchor.Match(d => d != default(DateTime), s => !s.IsNullOrEmpty());
-			if (!isValid) return string.Empty;
+			if (!IsValid) return string.Empty;
 
 			var separator = Self.Round.HasValue || Self.Ranges.HasAny() ? "||" : string.Empty;
 

src/Nest/QueryDsl/TermLevel/Range/DateRangeQuery .cs

@@ -43,10 +43,10 @@ public class DateRangeQuery : FieldNameQueryBase, IDateRangeQuery
 		internal override void InternalWrapInContainer(IQueryContainer c) => c.Range = this;
 
 		internal static bool IsConditionless(IDateRangeQuery q) => q.Field.IsConditionless()
-			|| q.GreaterThanOrEqualTo == null
-			&& q.LessThanOrEqualTo == null
-			&& q.GreaterThan == null
-			&& q.LessThan == null;
+			&& (q.GreaterThanOrEqualTo == null || !q.GreaterThanOrEqualTo.IsValid)
+			&& (q.LessThanOrEqualTo == null || !q.LessThanOrEqualTo.IsValid)
+			&& (q.GreaterThan == null || !q.GreaterThan.IsValid)
+			&& (q.LessThan == null || !q.LessThan.IsValid);
 	}
 
 	[JsonObject(MemberSerialization = MemberSerialization.OptIn)]

src/Tests/Tests/QueryDsl/TermLevel/Range/DateRangeQueryUsageTests.cs

@@ -66,4 +66,48 @@ protected override QueryContainer QueryFluent(QueryContainerDescriptor<Project>
 				.TimeZone("+01:00")
 			);
 	}
+
+	public class DateRangeDefaultDateTimeQueryUsageTests : QueryDslUsageTestsBase
+	{
+		public DateRangeDefaultDateTimeQueryUsageTests(ReadOnlyCluster cluster, EndpointUsage usage) : base(cluster, usage) { }
+
+		protected override ConditionlessWhen ConditionlessWhen => new ConditionlessWhen<IDateRangeQuery>(q => q.Range as IDateRangeQuery)
+			{ };
+
+		protected override QueryContainer QueryInitializer => new DateRangeQuery
+		{
+			Field = "description",
+			GreaterThan = default,
+			GreaterThanOrEqualTo = default,
+			LessThan = default,
+			LessThanOrEqualTo = default,
+			Format = "dd/MM/yyyy||yyyy",
+			TimeZone = "+01:00"
+		};
+
+		//TODO in 7.0 DateMath should make its DateTime nullable so that DateTime.MinValue is sent out as well.
+		// this is only marginally better then sending an empty string.
+		protected override object QueryJson => new
+		{
+			range = new
+			{
+				description = new
+				{
+					format = "dd/MM/yyyy||yyyy",
+					time_zone = "+01:00"
+				}
+			}
+		};
+
+		protected override QueryContainer QueryFluent(QueryContainerDescriptor<Project> q) => q
+			.DateRange(c => c
+				.Field(p => p.Description)
+				.GreaterThan(default)
+				.GreaterThanOrEquals(default)
+				.LessThan(default)
+				.LessThanOrEquals(default)
+				.Format("dd/MM/yyyy||yyyy")
+				.TimeZone("+01:00")
+			);
+	}
 }