diff --git a/Kronos.xcodeproj/project.pbxproj b/Kronos.xcodeproj/project.pbxproj
index 01c7de0..36c22e2 100644
--- a/Kronos.xcodeproj/project.pbxproj
+++ b/Kronos.xcodeproj/project.pbxproj
@@ -24,6 +24,7 @@
 		930B39DD2051E6D300360BA2 /* TimeStorage.swift in Sources */ = {isa = PBXBuildFile; fileRef = 930B39DC2051E6D300360BA2 /* TimeStorage.swift */; };
 		930B39E02051F26500360BA2 /* TimeStorageTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 930B39DE2051F25300360BA2 /* TimeStorageTests.swift */; };
 		C201748E1BD5509D00E4FE18 /* Kronos.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = C20174831BD5509D00E4FE18 /* Kronos.framework */; };
+		E5E7A7702BFE559F0025D9D0 /* UnfairLock.swift in Sources */ = {isa = PBXBuildFile; fileRef = E5E7A76F2BFE559F0025D9D0 /* UnfairLock.swift */; };
 /* End PBXBuildFile section */
 
 /* Begin PBXContainerItemProxy section */
@@ -59,6 +60,7 @@
 		C2C036D41C2B180D003FB853 /* UniversalFramework_Base.xcconfig */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.xcconfig; path = UniversalFramework_Base.xcconfig; sourceTree = "<group>"; };
 		C2C036D51C2B180D003FB853 /* UniversalFramework_Framework.xcconfig */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.xcconfig; path = UniversalFramework_Framework.xcconfig; sourceTree = "<group>"; };
 		C2C036D61C2B180D003FB853 /* UniversalFramework_Test.xcconfig */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.xcconfig; path = UniversalFramework_Test.xcconfig; sourceTree = "<group>"; };
+		E5E7A76F2BFE559F0025D9D0 /* UnfairLock.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = UnfairLock.swift; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -125,6 +127,7 @@
 				26447D7B1D6E54D400159BEE /* TimeFreeze.swift */,
 				930B39DC2051E6D300360BA2 /* TimeStorage.swift */,
 				5DB5A05F2BAAF67D0069CCF9 /* PrivacyInfo.xcprivacy */,
+				E5E7A76F2BFE559F0025D9D0 /* UnfairLock.swift */,
 			);
 			path = Sources;
 			sourceTree = "<group>";
@@ -247,6 +250,7 @@
 				26447D7E1D6E54D400159BEE /* InternetAddress.swift in Sources */,
 				26447D811D6E54D400159BEE /* NTPClient.swift in Sources */,
 				26447D7F1D6E54D400159BEE /* Data+Bytes.swift in Sources */,
+				E5E7A7702BFE559F0025D9D0 /* UnfairLock.swift in Sources */,
 				26447D841D6E54D400159BEE /* TimeFreeze.swift in Sources */,
 				26447D821D6E54D400159BEE /* NTPPacket.swift in Sources */,
 				26447D801D6E54D400159BEE /* NSTimer+ClosureKit.swift in Sources */,
diff --git a/Sources/Clock.swift b/Sources/Clock.swift
index fade988..b0c91a5 100644
--- a/Sources/Clock.swift
+++ b/Sources/Clock.swift
@@ -25,9 +25,20 @@ public typealias AnnotatedTime = (
 /// print(Clock.now)
 /// ```
 public struct Clock {
+    private static let lock = UnfairLock()
+
+    private static var _stableTime: TimeFreeze?
     private static var stableTime: TimeFreeze? {
-        didSet {
-            self.storage.stableTime = self.stableTime
+        get {
+            self.lock.around {
+                return self._stableTime
+            }
+        }
+        set {
+            self.lock.around {
+                self._stableTime = newValue
+                self.storage.stableTime = newValue
+            }
         }
     }
 
diff --git a/Sources/UnfairLock.swift b/Sources/UnfairLock.swift
new file mode 100644
index 0000000..47b01c5
--- /dev/null
+++ b/Sources/UnfairLock.swift
@@ -0,0 +1,29 @@
+import Foundation
+
+final class UnfairLock {
+    private let unfairLock: os_unfair_lock_t
+
+    init() {
+        unfairLock = .allocate(capacity: 1)
+        unfairLock.initialize(to: os_unfair_lock())
+    }
+
+    deinit {
+        unfairLock.deinitialize(count: 1)
+        unfairLock.deallocate()
+    }
+
+    func around<T>(_ block: () -> T) -> T {
+        lock()
+        defer { unlock() }
+        return block()
+    }
+
+    private func lock() {
+        os_unfair_lock_lock(unfairLock)
+    }
+
+    private func unlock() {
+        os_unfair_lock_unlock(unfairLock)
+    }
+}