diff --git a/src/libstd/sys/cloudabi/time.rs b/src/libstd/sys/cloudabi/time.rs
index 5e502dcb2ba50..b6496d65deaa4 100644
--- a/src/libstd/sys/cloudabi/time.rs
+++ b/src/libstd/sys/cloudabi/time.rs
@@ -25,7 +25,7 @@ impl Instant {
         }
     }
 
-    pub fn actually_monotonic() -> bool {
+    pub const fn actually_monotonic() -> bool {
         true
     }
 
diff --git a/src/libstd/sys/hermit/time.rs b/src/libstd/sys/hermit/time.rs
index 8372189546d07..c19de4643302a 100644
--- a/src/libstd/sys/hermit/time.rs
+++ b/src/libstd/sys/hermit/time.rs
@@ -128,7 +128,7 @@ impl Instant {
         Instant { t: Timespec::zero() }
     }
 
-    pub fn actually_monotonic() -> bool {
+    pub const fn actually_monotonic() -> bool {
         true
     }
 
diff --git a/src/libstd/sys/sgx/time.rs b/src/libstd/sys/sgx/time.rs
index 4659f7ba71fe0..2f43acd158ea8 100644
--- a/src/libstd/sys/sgx/time.rs
+++ b/src/libstd/sys/sgx/time.rs
@@ -26,7 +26,7 @@ impl Instant {
         Some(Instant(self.0.checked_sub(*other)?))
     }
 
-    pub fn actually_monotonic() -> bool {
+    pub const fn actually_monotonic() -> bool {
         false
     }
 
diff --git a/src/libstd/sys/unix/time.rs b/src/libstd/sys/unix/time.rs
index a9122defa5506..0cf0f4818ed60 100644
--- a/src/libstd/sys/unix/time.rs
+++ b/src/libstd/sys/unix/time.rs
@@ -158,7 +158,7 @@ mod inner {
             Instant { t: 0 }
         }
 
-        pub fn actually_monotonic() -> bool {
+        pub const fn actually_monotonic() -> bool {
             true
         }
 
@@ -308,11 +308,21 @@ mod inner {
             }
         }
 
-        pub fn actually_monotonic() -> bool {
-            (cfg!(target_os = "linux") && cfg!(target_arch = "x86_64")) ||
-            (cfg!(target_os = "linux") && cfg!(target_arch = "x86")) ||
-            cfg!(target_os = "fuchsia") ||
-            false // last clause, used so `||` is always trailing above
+        pub const fn actually_monotonic() -> bool {
+            // note: duplicate attributes may be optimized when
+            // conditional expressions in constant functions hits stable.
+            #[cfg(any(
+                all(target_os = "linux", target_arch = "x86_64"),
+                all(target_os = "linux", target_arch = "x86"),
+                all(target_os = "fuchsia"),
+            ))]
+            { true }
+            #[cfg(not(any(
+                all(target_os = "linux", target_arch = "x86_64"),
+                all(target_os = "linux", target_arch = "x86"),
+                all(target_os = "fuchsia"),
+            )))]
+            { false }
         }
 
         pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
diff --git a/src/libstd/sys/vxworks/time.rs b/src/libstd/sys/vxworks/time.rs
index cb3a4241ea601..4089751ce1b3c 100644
--- a/src/libstd/sys/vxworks/time.rs
+++ b/src/libstd/sys/vxworks/time.rs
@@ -147,7 +147,7 @@ mod inner {
             }
         }
 
-        pub fn actually_monotonic() -> bool {
+        pub const fn actually_monotonic() -> bool {
             true
         }
 
diff --git a/src/libstd/sys/wasi/time.rs b/src/libstd/sys/wasi/time.rs
index 4394a22f9c233..3d759ecd7cada 100644
--- a/src/libstd/sys/wasi/time.rs
+++ b/src/libstd/sys/wasi/time.rs
@@ -29,7 +29,7 @@ impl Instant {
         Instant(Duration::from_secs(0))
     }
 
-    pub fn actually_monotonic() -> bool {
+    pub const fn actually_monotonic() -> bool {
         true
     }
 
diff --git a/src/libstd/sys/wasm/time.rs b/src/libstd/sys/wasm/time.rs
index dd9ad3760b050..180f088b54f27 100644
--- a/src/libstd/sys/wasm/time.rs
+++ b/src/libstd/sys/wasm/time.rs
@@ -17,7 +17,7 @@ impl Instant {
         Instant(Duration::from_secs(0))
     }
 
-    pub fn actually_monotonic() -> bool {
+    pub const fn actually_monotonic() -> bool {
         false
     }
 
diff --git a/src/libstd/sys/windows/time.rs b/src/libstd/sys/windows/time.rs
index bd533c93d434c..4f540df3e6ed4 100644
--- a/src/libstd/sys/windows/time.rs
+++ b/src/libstd/sys/windows/time.rs
@@ -41,7 +41,7 @@ impl Instant {
         perf_counter::PerformanceCounterInstant::now().into()
     }
 
-    pub fn actually_monotonic() -> bool {
+    pub const fn actually_monotonic() -> bool {
         false
     }