clock-handling

Clock handling during installation
==================================

For background and terminology, see https://wiki.ubuntu.com/HardwareClock.

The installer needs to be careful to handle the clock in ways that do not
cause problems for the installed system, particularly when the hardware
clock is kept in local time (cf.
http://www.cl.cam.ac.uk/~mgk25/mswish/ut-rtc.html).

We should adhere to the following rules:

  * Any changes to the clock should be made before writing anything to disk.

    This rule ensures that filesystem superblock timestamps are written
    using a consistent system wall clock. If the clock is changed after disk
    writes, then the superblock timestamp may appear to be in the future
    after reboot.

  * The hardware clock should be set using a consistent configuration.

    Noting that the hardware clock is typically saved on live CD shutdown,
    the live CD installer should ensure that the UTC= configuration in the
    live system is consistent with that in the target system, so that
    hwclock is not inadvertently run multiple times with different
    configurations.

  * If possible, the user should be asked to confirm the system wall clock
    time, and encouraged to reset it if it is incorrect.

    This is a good idea on general principles, particularly when NTP is not
    available. It's better for the time to be set correctly as early as
    possible.

  * The installer should make an effort to minimise the magnitude of changes
    to the hardware clock.

    While the installer attempts to guess hardware clock configuration based
    on which other operating systems installed, this kind of approach is not
    and cannot be entirely accurate. If NTP is available, then after
    timezone selection it is possible to compare the hardware clock against
    the system wall clock offset by timezone. If local time appears to be
    a better guess than UTC, then it would be reasonable to use that.

(As of version 1.99.24, note that Ubiquity only follows the first two of
these rules.)

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