diff --git a/.github/workflows/api.yml b/.github/workflows/api.yml
index 2655cb8..189f232 100644
--- a/.github/workflows/api.yml
+++ b/.github/workflows/api.yml
@@ -18,6 +18,9 @@ jobs:
     - name: Mark Workspace As Safe
       # https://github.com/actions/checkout/issues/766
       run: git config --global --add safe.directory ${GITHUB_WORKSPACE}
+    - name: Swift Version
+      run: |
+        swift --version
     - name: Diagnose API Breaking Changes
       run: |
         swift package diagnose-api-breaking-changes origin/main --products CodableDatastore
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index a57c24d..e3a0c60 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -12,9 +12,12 @@ jobs:
     steps:
     - name: Checkout Source
       uses: actions/checkout@v3
+    - name: Swift Version
+      run: |
+        swift --version
     - name: Build
       run: |
-        swift build --build-tests --configuration release -Xswiftc -enable-testing -Xswiftc -warnings-as-errors -Xcc -Werror
+        swift build --build-tests --configuration release -Xswiftc -enable-testing -Xswiftc -warnings-as-errors
     - name: Run Tests
       run: |
         swift test --skip-build --configuration release
@@ -25,9 +28,12 @@ jobs:
     steps:
     - name: Checkout Source
       uses: actions/checkout@v3
+    - name: Swift Version
+      run: |
+        swift --version
     - name: Build
       run: |
-        swift build --build-tests --configuration debug -Xswiftc -enable-testing -Xswiftc -warnings-as-errors -Xcc -Werror
+        swift build --build-tests --configuration debug -Xswiftc -enable-testing -Xswiftc -warnings-as-errors
     - name: Run Tests
       run: |
         swift test --skip-build --configuration debug
diff --git a/Sources/CodableDatastore/Datastore/Datastore.swift b/Sources/CodableDatastore/Datastore/Datastore.swift
index 6645143..aa40082 100644
--- a/Sources/CodableDatastore/Datastore/Datastore.swift
+++ b/Sources/CodableDatastore/Datastore/Datastore.swift
@@ -194,7 +194,7 @@ extension Datastore {
         
         /// Notify progress handlers we are evaluating for possible migrations.
         for handler in warmupProgressHandlers {
-            handler(.evaluating)
+            await handler(.evaluating)
         }
         
         /// Grab an up-to-date descriptor and check the indexes against it
@@ -272,7 +272,7 @@ extension Datastore {
                 
                 /// Notify progress handlers we are starting an entry.
                 for handler in warmupProgressHandlers {
-                    handler(.working(current: index, total: persistedDescriptor.size))
+                    await handler(.working(current: index, total: persistedDescriptor.size))
                 }
                 
                 let instanceData = try await encoder(instance)
@@ -354,7 +354,7 @@ extension Datastore {
         let completeProgress = Progress.complete(total: persistedDescriptor.size)
         
         for handler in warmupProgressHandlers {
-            handler(completeProgress)
+            await handler(completeProgress)
         }
         
         warmupProgressHandlers.removeAll()
@@ -401,7 +401,7 @@ extension Datastore where AccessMode == ReadWrite {
             else { return }
             
             let warmUpProgress = try await self.warmupIfNeeded { progress in
-                progressHandler?(progress.adding(current: 0, total: descriptor.size))
+                await progressHandler?(progress.adding(current: 0, total: descriptor.size))
             }
             
             /// Make sure we still need to do the work, as the warm up may have made changes anyways due to incompatible types.
@@ -419,12 +419,12 @@ extension Datastore where AccessMode == ReadWrite {
                 /// Make sure the stored version is smaller than the one we require, otherwise stop early.
                 version.rawValue < minimumVersion.rawValue
             else {
-                progressHandler?(warmUpProgress.adding(current: descriptor.size, total: descriptor.size))
+                await progressHandler?(warmUpProgress.adding(current: descriptor.size, total: descriptor.size))
                 return
             }
             
             try await self.migrate(index: index) { migrateProgress in
-                progressHandler?(warmUpProgress.adding(migrateProgress))
+                await progressHandler?(warmUpProgress.adding(migrateProgress))
             }
         }
     }
diff --git a/Sources/CodableDatastore/Datastore/Progress.swift b/Sources/CodableDatastore/Datastore/Progress.swift
index 6823c4e..a01d1e5 100644
--- a/Sources/CodableDatastore/Datastore/Progress.swift
+++ b/Sources/CodableDatastore/Datastore/Progress.swift
@@ -8,7 +8,7 @@
 
 import Foundation
 
-public typealias ProgressHandler = @Sendable (_ progress: Progress) -> Void
+public typealias ProgressHandler = @Sendable (_ progress: Progress) async -> Void
 
 public enum Progress: Sendable {
     case evaluating
diff --git a/Sources/CodableDatastore/Indexes/Indexable.swift b/Sources/CodableDatastore/Indexes/Indexable.swift
index 68af36c..ea7b881 100644
--- a/Sources/CodableDatastore/Indexes/Indexable.swift
+++ b/Sources/CodableDatastore/Indexes/Indexable.swift
@@ -24,6 +24,17 @@ public struct AnyIndexable {
 
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
 /// Matching implementation from https://github.com/apple/swift/pull/64899/files
+#if compiler(>=6)
+extension Never: @retroactive Codable {
+    public init(from decoder: any Decoder) throws {
+        let context = DecodingError.Context(
+            codingPath: decoder.codingPath,
+            debugDescription: "Unable to decode an instance of Never.")
+        throw DecodingError.typeMismatch(Never.self, context)
+    }
+    public func encode(to encoder: any Encoder) throws {}
+}
+#else
 extension Never: Codable {
     public init(from decoder: any Decoder) throws {
         let context = DecodingError.Context(
@@ -34,6 +45,7 @@ extension Never: Codable {
     public func encode(to encoder: any Encoder) throws {}
 }
 #endif
+#endif
 
 /// A marker protocol for types that can be used as a ranged index.
 ///
@@ -100,7 +112,7 @@ extension Optional: RangedIndexable where Wrapped: RangedIndexable {}
 // MARK: - Foundation Conformances
 
 extension Date: RangedIndexable {}
-#if canImport(Darwin)
+#if canImport(Darwin) || compiler(>=6.2)
 extension Decimal: RangedIndexable {}
 #else
 extension Decimal: RangedIndexable, @unchecked Sendable {}
diff --git a/Sources/CodableDatastore/Persistence/AccessMode.swift b/Sources/CodableDatastore/Persistence/AccessMode.swift
index 44ede4f..18a8814 100644
--- a/Sources/CodableDatastore/Persistence/AccessMode.swift
+++ b/Sources/CodableDatastore/Persistence/AccessMode.swift
@@ -7,7 +7,7 @@
 //
 
 /// An AccessMode marker type.
-public protocol _AccessMode {}
+public protocol _AccessMode: Sendable {}
 
 /// A marker type that indicates read-only access.
 public enum ReadOnly: _AccessMode {}
diff --git a/Sources/CodableDatastore/Persistence/Disk Persistence/Datastore/DatastorePage.swift b/Sources/CodableDatastore/Persistence/Disk Persistence/Datastore/DatastorePage.swift
index 30884dc..9e26972 100644
--- a/Sources/CodableDatastore/Persistence/Disk Persistence/Datastore/DatastorePage.swift	
+++ b/Sources/CodableDatastore/Persistence/Disk Persistence/Datastore/DatastorePage.swift	
@@ -216,7 +216,7 @@ actor MultiplexedAsyncSequence<Base: AsyncSequence & Sendable>: AsyncSequence wh
     }
 }
 
-extension RangeReplaceableCollection {
+extension RangeReplaceableCollection where Self: Sendable {
     init<S: AsyncSequence>(_ sequence: S) async throws where S.Element == Element {
         self = try await sequence.reduce(into: Self.init()) { @Sendable partialResult, element in
             partialResult.append(element)
diff --git a/Sources/CodableDatastore/Persistence/Disk Persistence/DatedIdentifier.swift b/Sources/CodableDatastore/Persistence/Disk Persistence/DatedIdentifier.swift
index 58e2712..4abfbd4 100644
--- a/Sources/CodableDatastore/Persistence/Disk Persistence/DatedIdentifier.swift	
+++ b/Sources/CodableDatastore/Persistence/Disk Persistence/DatedIdentifier.swift	
@@ -120,6 +120,6 @@ private extension DateFormatter {
     }()
 }
 
-#if !canImport(Darwin)
+#if !canImport(Darwin) && compiler(<6.2)
 extension DateFormatter: @unchecked Sendable {}
 #endif
diff --git a/Sources/CodableDatastore/Persistence/Disk Persistence/ISO8601DateFormatter+Milliseconds.swift b/Sources/CodableDatastore/Persistence/Disk Persistence/ISO8601DateFormatter+Milliseconds.swift
index dd85821..be7c483 100644
--- a/Sources/CodableDatastore/Persistence/Disk Persistence/ISO8601DateFormatter+Milliseconds.swift	
+++ b/Sources/CodableDatastore/Persistence/Disk Persistence/ISO8601DateFormatter+Milliseconds.swift	
@@ -8,8 +8,29 @@
 
 import Foundation
 
-extension ISO8601DateFormatter {
-    static let withMilliseconds: ISO8601DateFormatter = {
+private struct GlobalDateFormatter: Sendable {
+    @available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)
+    static let cachedFormatter = Date.ISO8601FormatStyle(includingFractionalSeconds: true)
+    
+    static let parse: @Sendable (_ value: String) -> Date? = {
+        if #available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *) {
+            return { try? cachedFormatter.parse($0) }
+        } else {
+            return { ISO8601DateFormatter.withMilliseconds.date(from: $0) }
+        }
+    }()
+    
+    static let format: @Sendable (_ value: Date) -> String = {
+        if #available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *) {
+            return { cachedFormatter.format($0) }
+        } else {
+            return { ISO8601DateFormatter.withMilliseconds.string(from: $0) }
+        }
+    }()
+}
+
+private extension ISO8601DateFormatter {
+    nonisolated(unsafe) static let withMilliseconds: ISO8601DateFormatter = {
         let formatter = ISO8601DateFormatter()
         formatter.timeZone = TimeZone(secondsFromGMT: 0)
         formatter.formatOptions = [
@@ -27,7 +48,7 @@ extension JSONDecoder.DateDecodingStrategy {
     static let iso8601WithMilliseconds: Self = custom { decoder in
         let container = try decoder.singleValueContainer()
         let string = try container.decode(String.self)
-        guard let date = ISO8601DateFormatter.withMilliseconds.date(from: string) else {
+        guard let date = GlobalDateFormatter.parse(string) else {
             throw DecodingError.dataCorruptedError(in: container, debugDescription: "Invalid date: \(string)")
         }
         return date
@@ -37,13 +58,13 @@ extension JSONDecoder.DateDecodingStrategy {
 
 extension JSONEncoder.DateEncodingStrategy {
     static let iso8601WithMilliseconds: Self = custom { date, encoder in
-        let string = ISO8601DateFormatter.withMilliseconds.string(from: date)
+        let string = GlobalDateFormatter.format(date)
         var container = encoder.singleValueContainer()
         try container.encode(string)
     }
 }
 
-#if compiler(>=6)
+#if compiler(>=6) && compiler(<6.2)
 extension ISO8601DateFormatter: @unchecked @retroactive Sendable {}
 extension JSONDecoder.DateDecodingStrategy: @unchecked Sendable {}
 extension JSONEncoder.DateEncodingStrategy: @unchecked Sendable {}
diff --git a/Sources/CodableDatastore/Persistence/Disk Persistence/JSONCoder.swift b/Sources/CodableDatastore/Persistence/Disk Persistence/JSONCoder.swift
index df81289..5f31b4c 100644
--- a/Sources/CodableDatastore/Persistence/Disk Persistence/JSONCoder.swift	
+++ b/Sources/CodableDatastore/Persistence/Disk Persistence/JSONCoder.swift	
@@ -29,7 +29,7 @@ extension JSONDecoder {
     }()
 }
 
-#if !canImport(Darwin)
+#if !canImport(Darwin) && compiler(<6.2)
 extension JSONEncoder: @unchecked Sendable {}
 extension JSONDecoder: @unchecked Sendable {}
 #endif