diff --git a/src/SwaggerProvider.Runtime/RuntimeHelpers.fs b/src/SwaggerProvider.Runtime/RuntimeHelpers.fs
index c7f07e3a..bf709f98 100644
--- a/src/SwaggerProvider.Runtime/RuntimeHelpers.fs
+++ b/src/SwaggerProvider.Runtime/RuntimeHelpers.fs
@@ -376,9 +376,16 @@ module RuntimeHelpers =
 
                 if vTy = typeof<string> then
                     sb.Append('"').Append(v.ToString()).Append('"') |> ignore
+                elif vTy.FullName = dateOnlyTypeName then
+                    sb.Append(formatDateOrTimeValue "yyyy-MM-dd" vTy v) |> ignore
+                elif vTy.FullName = timeOnlyTypeName then
+                    sb.Append(formatDateOrTimeValue "HH:mm:ss.FFFFFFF" vTy v) |> ignore
                 elif vTy.IsArray then
                     sb.Append('[') |> ignore
                     let mutable firstEl = true
+                    let elTy = vTy.GetElementType()
+                    let isDateOnly = not(isNull elTy) && elTy.FullName = dateOnlyTypeName
+                    let isTimeOnly = not(isNull elTy) && elTy.FullName = timeOnlyTypeName
 
                     for x in (v :?> Array) |> Seq.cast<obj> do
                         if not firstEl then
@@ -389,7 +396,14 @@ module RuntimeHelpers =
                         if isNull x then
                             sb.Append("null") |> ignore
                         else
-                            sb.Append(x.ToString()) |> ignore
+                            let xTy = x.GetType()
+
+                            if isDateOnly then
+                                sb.Append(formatDateOrTimeValue "yyyy-MM-dd" xTy x) |> ignore
+                            elif isTimeOnly then
+                                sb.Append(formatDateOrTimeValue "HH:mm:ss.FFFFFFF" xTy x) |> ignore
+                            else
+                                sb.Append(x.ToString()) |> ignore
 
                     sb.Append(']') |> ignore
                 else
diff --git a/tests/SwaggerProvider.Tests/RuntimeHelpersTests.fs b/tests/SwaggerProvider.Tests/RuntimeHelpersTests.fs
index 14ed60c0..5dfa7057 100644
--- a/tests/SwaggerProvider.Tests/RuntimeHelpersTests.fs
+++ b/tests/SwaggerProvider.Tests/RuntimeHelpersTests.fs
@@ -1128,6 +1128,18 @@ type FmtMulti(age: int, name: string) =
 type FmtArray(tags: string[]) =
     member _.Tags = tags
 
+type FmtDateOnly(date: DateOnly) =
+    member _.Date = date
+
+type FmtTimeOnly(time: TimeOnly) =
+    member _.Time = time
+
+type FmtDateOnlyArray(dates: DateOnly[]) =
+    member _.Dates = dates
+
+type FmtTimeOnlyArray(times: TimeOnly[]) =
+    member _.Times = times
+
 
 module FormatObjectTests =
 
@@ -1177,6 +1189,31 @@ module FormatObjectTests =
         let obj = FmtMulti(30, "Bob")
         formatObject obj |> shouldEqual "{Age=30; Name=\"Bob\"}"
 
+    [<Fact>]
+    let ``formatObject formats DateOnly property as ISO 8601``() =
+        let obj = FmtDateOnly(DateOnly(2025, 7, 4))
+        formatObject obj |> shouldEqual "{Date=2025-07-04}"
+
+    [<Fact>]
+    let ``formatObject formats TimeOnly property using HH:mm:ss.FFFFFFF``() =
+        let obj = FmtTimeOnly(TimeOnly(14, 30, 0))
+        formatObject obj |> shouldEqual "{Time=14:30:00}"
+
+    [<Fact>]
+    let ``formatObject formats TimeOnly property with sub-second precision``() =
+        let obj = FmtTimeOnly(TimeOnly(9, 5, 3, 123))
+        formatObject obj |> shouldEqual "{Time=09:05:03.123}"
+
+    [<Fact>]
+    let ``formatObject formats DateOnly array elements as ISO 8601``() =
+        let obj = FmtDateOnlyArray([| DateOnly(2025, 1, 1); DateOnly(2025, 12, 31) |])
+        formatObject obj |> shouldEqual "{Dates=[2025-01-01; 2025-12-31]}"
+
+    [<Fact>]
+    let ``formatObject formats TimeOnly array elements using HH:mm:ss.FFFFFFF``() =
+        let obj = FmtTimeOnlyArray([| TimeOnly(8, 0, 0); TimeOnly(14, 30, 45, 500) |])
+        formatObject obj |> shouldEqual "{Times=[08:00:00; 14:30:45.5]}"
+
 
 module ToFormUrlEncodedContentTests =