ESPGeiger

NTP is for timekeeping indeed. At present it sets to UTC, I'm thinking about supporting timezones, but see little need at the moment. NTP is synched by the underlying ESP functionality with mostly default settings, I believe this is set to sync at roughly 1 hourly intervals.

NTP cannot be obtained via DHCP like IP addresses, so NTP has to be defined. ntp.org (defacto internet-wide NTP provider) returns servers in your local region, so time is synchronised as locally as possible.

This can be set at build time - if you were to build your own image - but not at runtime currently. An option for setting the NTP server could be feasible but not sure on the usefulness of this vs other features at present.

Not running an NTP server at home, though my router has the functionality.
For my purposes, the espgeiger doesn't even need to know the time as Grafana takes care of that.