[#3501375] Implement PSR-20 clock

Closes #3501375

composer/Metapackage/CoreRecommended/composer.json

@@ -25,6 +25,7 @@
         "pear/pear_exception": "~v1.0.2",
         "php-tuf/composer-stager": "~v2.0.0",
         "psr/cache": "~3.0.0",
+        "psr/clock": "~1.0.0",
         "psr/container": "~2.0.2",
         "psr/event-dispatcher": "~1.0.0",
         "psr/http-client": "~1.0.3",