diff --git a/src/shims/foreign_items/posix/macos.rs b/src/shims/foreign_items/posix/macos.rs
index 34661fb2383..0bb4710769d 100644
--- a/src/shims/foreign_items/posix/macos.rs
+++ b/src/shims/foreign_items/posix/macos.rs
@@ -66,6 +66,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 let result = this.gettimeofday(args[0], args[1])?;
                 this.write_scalar(Scalar::from_int(result, dest.layout.size), dest)?;
             }
+            "mach_absolute_time" => {
+                let result = this.mach_absolute_time()?;
+                this.write_scalar(Scalar::from_uint(result, dest.layout.size), dest)?;
+            }
 
             // Other shims
             "pthread_attr_get_np" => {
diff --git a/src/shims/fs.rs b/src/shims/fs.rs
index cb429109a4a..a5aae5ed90c 100644
--- a/src/shims/fs.rs
+++ b/src/shims/fs.rs
@@ -574,8 +574,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         buf_op: OpTy<'tcx, Tag>,
     ) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();
-        this.check_no_isolation("stat")?;
         this.assert_platform("macos", "stat");
+        this.check_no_isolation("stat")?;
         // `stat` always follows symlinks.
         this.macos_stat_or_lstat(true, path_op, buf_op)
     }
@@ -587,8 +587,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         buf_op: OpTy<'tcx, Tag>,
     ) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();
-        this.check_no_isolation("lstat")?;
         this.assert_platform("macos", "lstat");
+        this.check_no_isolation("lstat")?;
         this.macos_stat_or_lstat(false, path_op, buf_op)
     }
 
@@ -599,8 +599,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
     ) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();
 
-        this.check_no_isolation("fstat")?;
         this.assert_platform("macos", "fstat");
+        this.check_no_isolation("fstat")?;
 
         let fd = this.read_scalar(fd_op)?.to_i32()?;
 
@@ -621,8 +621,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
     ) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();
 
-        this.check_no_isolation("statx")?;
         this.assert_platform("linux", "statx");
+        this.check_no_isolation("statx")?;
 
         let statxbuf_scalar = this.read_scalar(statxbuf_op)?.not_undef()?;
         let pathname_scalar = this.read_scalar(pathname_op)?.not_undef()?;
@@ -880,8 +880,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
     ) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();
 
-        this.check_no_isolation("readdir64_r")?;
         this.assert_platform("linux", "readdir64_r");
+        this.check_no_isolation("readdir64_r")?;
 
         let dirp = this.read_scalar(dirp_op)?.to_machine_usize(this)?;
 
@@ -967,8 +967,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
     ) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();
 
-        this.check_no_isolation("readdir_r")?;
         this.assert_platform("macos", "readdir_r");
+        this.check_no_isolation("readdir_r")?;
 
         let dirp = this.read_scalar(dirp_op)?.to_machine_usize(this)?;
 
diff --git a/src/shims/time.rs b/src/shims/time.rs
index 2d812208fba..b270c9770f8 100644
--- a/src/shims/time.rs
+++ b/src/shims/time.rs
@@ -1,4 +1,5 @@
 use std::time::{Duration, SystemTime, Instant};
+use std::convert::TryFrom;
 
 use crate::stacked_borrows::Tag;
 use crate::*;
@@ -12,7 +13,6 @@ pub fn system_time_to_duration<'tcx>(time: &SystemTime) -> InterpResult<'tcx, Du
 
 impl<'mir, 'tcx> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
-    // Foreign function used by linux
     fn clock_gettime(
         &mut self,
         clk_id_op: OpTy<'tcx, Tag>,
@@ -20,8 +20,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
     ) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();
 
+        this.assert_platform("linux", "clock_gettime");
         this.check_no_isolation("clock_gettime")?;
-        this.assert_platform("linux");
 
         let clk_id = this.read_scalar(clk_id_op)?.to_i32()?;
         let tp = this.deref_operand(tp_op)?;
@@ -50,7 +50,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
         Ok(0)
     }
-    // Foreign function used by generic unix (in particular macOS)
+
     fn gettimeofday(
         &mut self,
         tv_op: OpTy<'tcx, Tag>,
@@ -58,8 +58,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
     ) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();
 
+        this.assert_platform("macos", "gettimeofday");
         this.check_no_isolation("gettimeofday")?;
-        this.assert_platform("macos");
 
         // Using tz is obsolete and should always be null
         let tz = this.read_scalar(tz_op)?.not_undef()?;
@@ -84,4 +84,17 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
         Ok(0)
     }
+
+    fn mach_absolute_time(&self) -> InterpResult<'tcx, u64> {
+        let this = self.eval_context_ref();
+
+        this.assert_platform("macos", "mach_absolute_time");
+        this.check_no_isolation("mach_absolute_time")?;
+
+        // This returns a u64, with time units determined dynamically by `mach_timebase_info`.
+        // We return plain nanoseconds.
+        let duration = Instant::now().duration_since(this.machine.time_anchor);
+        u64::try_from(duration.as_nanos())
+            .map_err(|_| err_unsup_format!("programs running longer than 2^64 nanoseconds are not supported").into())
+    }
 }