diff --git a/composer.json b/composer.json
index 57cbd22..e1886d4 100644
--- a/composer.json
+++ b/composer.json
@@ -34,7 +34,7 @@
         "ext-spl": "*",
         "ext-xmlreader": "*",
 
-        "simplesamlphp/assert": "^1.1"
+        "simplesamlphp/assert": "dev-feature/xs-datetime-regex"
     },
     "require-dev": {
         "simplesamlphp/simplesamlphp-test-framework": "^1.6"