Hi there, time updates are done on GrapheneOS in a more secure way than on stock Android. For more details, please see the following details from the FAQs. It's security sensitive to keep the time set accurately, and therefore it's recommended you enable the toggle to update the time automatically.
An HTTPS connection is made to https://time.grapheneos.org/generate_204 to update the time with a millisecond precision X-Time header. As part of future support for using other services, it falls back to the standard Date header with second precision.
This is a full replacement for Android's standard network time update implementation, which uses unauthentication SNTP (Simple Network Time Protocol) with fallback to the cellular network when it's not available (GNSS can also be used as a time source but is disabled by default, and OEMs can choose the priority order). Network time updates are security sensitive since certificate validation depends on having an accurate time, but the standard NTP / SNTP protocols used across most OSes have no authentication or encryption.
We plan to offer a toggle to use the standard functionality instead of HTTPS-based time updates in order to blend in with other devices.
Network time can be disabled with the toggle at Settings ➔ System ➔ Date & time ➔ Set time automatically. Unlike AOSP or the stock OS on the supported devices, GrapheneOS stops making network time connections when using network time is disabled rather than just not setting the clock based on it. The time zone is still obtained directly via the time zone provided by the mobile network (NITZ) when available which you can also disable by the "Set time zone automatically" toggle.
https://grapheneos.org/faq#:~:text=An%20HTTPS%20connection,NTP%20/%20SNTP%20protocols