Fix: MS Access DateTime parameter handling with milliseconds

PetaPoco.Tests.Integration/Databases/MSAccessTests/MSAccessInsertTests.cs

@@ -1,3 +1,6 @@
+using System;
+using PetaPoco.Tests.Integration.Models;
+using Shouldly;
 using Xunit;
 using PetaPoco.Tests.Integration.Providers;
 
@@ -11,5 +14,52 @@ public MSAccessInsertTests()
             : base(new MSAccessTestProvider())
         {
         }
+
+        /// <summary>
+        /// Regression test for MS Access DateTime millisecond handling.
+        /// MS Access OLE DB provider cannot handle DateTime parameters with sub-second precision.
+        /// Without proper handling in MapParameterValue, DateTime values with milliseconds cause
+        /// "Data type mismatch in criteria expression" errors. The fix truncates milliseconds before
+        /// sending DateTime values to the OLE DB provider.
+        /// See: https://github.com/CollaboratingPlatypus/PetaPoco/issues/736
+        /// </summary>
+        [Fact]
+        public void Insert_GivenPocoWithMillisecondPrecisionDateTime_ShouldInsertSuccessfully()
+        {
+            // Test with DateTime that has NO milliseconds (baseline)
+            var personWithoutMillis = new Person
+            {
+                Id = Guid.NewGuid(),
+                Name = "No Milliseconds Person",
+                Age = 35,
+                Height = 180,
+                Dob = new DateTime(2000, 6, 15, 14, 30, 45, 0, DateTimeKind.Utc)
+            };
+
+            _ = DB.Insert(personWithoutMillis);
+            var retrieved1 = DB.Single<Person>(personWithoutMillis.Id);
+            retrieved1.ShouldNotBeNull();
+            retrieved1.Dob.ShouldNotBeNull();
+
+            // Test with DateTime that HAS milliseconds (the actual regression test)
+            // Before fix: throws OleDbException "Data type mismatch in criteria expression"
+            // After fix: succeeds but milliseconds are truncated
+            var personWithMillis = new Person
+            {
+                Id = Guid.NewGuid(),
+                Name = "With Milliseconds Person",
+                Age = 40,
+                Height = 185,
+                Dob = new DateTime(2000, 6, 15, 14, 30, 45, 123, DateTimeKind.Utc)
+            };
+
+            _ = DB.Insert(personWithMillis);
+
+            var retrieved2 = DB.Single<Person>(personWithMillis.Id);
+            retrieved2.ShouldNotBeNull();
+            retrieved2.Dob.ShouldNotBeNull();
+            // Allow 1 second tolerance because milliseconds are truncated by the fix
+            retrieved2.Dob.Value.ShouldBe(personWithMillis.Dob.Value, TimeSpan.FromSeconds(1));
+        }
     }
 }

PetaPoco/Providers/MSAccessDatabaseProvider.cs

@@ -48,6 +48,15 @@ public override object MapParameterValue(object value)
                     return (int)ul;
             }
 
+            // MS Access OLE DB provider cannot handle DateTime parameters with sub-second precision (milliseconds).
+            // Truncate to whole seconds to avoid "Data type mismatch in criteria expression" errors.
+            // See: https://github.com/CollaboratingPlatypus/PetaPoco/issues/736
+            if (value is DateTime dt)
+            {
+                // Remove milliseconds by creating new DateTime with only year/month/day/hour/minute/second
+                return new DateTime(dt.Year, dt.Month, dt.Day, dt.Hour, dt.Minute, dt.Second, dt.Kind);
+            }
+
             // Let base handle other common conversions (eg: bool -> int)
             return base.MapParameterValue(value);
         }