diff --git a/rfcs/testdriver_set_time_zone.md b/rfcs/testdriver_set_time_zone.md
new file mode 100644
index 00000000..38baab6c
--- /dev/null
+++ b/rfcs/testdriver_set_time_zone.md
@@ -0,0 +1,21 @@
+# RFC 73 : Add testdriver.js support for the `timeZone` WebDriver command.
+
+## Summary
+Add testdriver.js support for the `timeZone` WebDriver extension, which will
+allow setting the current time zone in the browser. This extension is specced
+as part of the [HTML spec](https://github.com/whatwg/html/pull/3047).
+
+## Details
+Changing the time zone is not available via normal web APIs. To test the effect
+of changing time zone on the `timezonechange` event there needs to be a way
+to instruct the browser to act as if the time zone is changing: this WebDriver
+extension provides that ability.
+
+## Risks
+* The WebDriver endpoint is new and may still require changes, which would then
+  affect the testdriver.js API as well.
+* The WebDriver endpoint is very specific and will likely only be used for a
+  small set of tests, so the maintenance to value ratio may be low.
+    * That said, testdriver.js is designed to make supporting a command
+      low-maintenance, and to not require every executor to support every
+      testdriver.js command.