# Datetime & Timezone Tests Code that uses the current system datetime needs special testing to validate your date math. In Unit Testing, there are two features we use regularly for this. {% hint style="info" %} Javascript's Datetime libraries are very difficult to work with effectively. We highly recommend that you use luxon instead. {% endhint %} ## System Clock Changes When your code uses the browser's system clock for calculations, such as countdown / time-until-event calculations, you can use Jest's system clock change capabilities. ## Timezone Changes Timezone changes can also be made prior to the test being run, for example; ``` TZ=Pacific/Auckland npm test -- --silent=false ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://engine.sygnal.com/usage/unit-testing/datetime-and-timezone-tests.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.