While it's a good idea to use performance.now() for it's better precision* and monotonic guarantee, it's not really a huge concern for this kind of application

While it's a good idea to use performance.now() for it's better precision* and monotonic guarantee, it's not really a huge concern for this kind of application (measuring time between A and B on the client). You're extremely unlikely to experience clock skew during those brief windows, and the entire web relied on Date.now for performance monitoring for decades. For dealing with timestamps reliably on the client, we'll just instantiate all dates based on server time instead. * at least in FF it has been rolled back to 1ms resolution due to privacy/fingerprinting concerns