diff --git a/README.md b/README.md
index 1d63ba5..be9794d 100644
--- a/README.md
+++ b/README.md
@@ -285,6 +285,7 @@ time | `(time)` | `1592011969` | Return the current unix timestamp using [time](
 date | `(date "Y-m-d H:i:s")` | `"2020-06-13 08:33:29"` | Format the current time and date using [date](https://www.php.net/manual/en/function.date.php).
 strtotime | `(strtotime "2020-06-13 08:34:47")` | `1592012087` | Parse datetime string into unix timestamp using [strtotime](https://www.php.net/manual/en/function.strtotime.php).
 sleep | `(sleep 2000)` | `null` | Sleep for the given period given in milliseconds using [usleep](https://www.php.net/manual/en/function.usleep).
+timer | `(timer (fn (d) (sleep d)) 200)` | `0.20010209` | Measure the execution time of a function and return it in seconds.
 
 ### Type functions
 
diff --git a/src/Lib/Time.php b/src/Lib/Time.php
index 908ee66..6a7c2ca 100644
--- a/src/Lib/Time.php
+++ b/src/Lib/Time.php
@@ -3,6 +3,7 @@ namespace MadLisp\Lib;
 
 use MadLisp\CoreFunc;
 use MadLisp\Env;
+use MadLisp\Func;
 
 class Time implements ILib
 {
@@ -26,5 +27,14 @@ class Time implements ILib
                 return null;
             }
         ));
+
+        $env->set('timer', new CoreFunc('timer', 'Measure the execution time of a function and return it in seconds.', 1, -1,
+            function (Func $f, ...$args) {
+                $start = microtime(true);
+                $f->call($args);
+                $end = microtime(true);
+                return $end - $start;
+            }
+        ));
     }
 }