Khal cannot parse ISO8601 from system default

Describe the bug
While Khal seems to be able to pick up my time format from my locale.conf, it doesn't seem to be able to parse it without defining the locale manually.

To Reproduce
Steps to reproduce the behavior:

• Set your locale so that ISO8601 is selected as a time format in /etc/locale.conf, e.g.:

LANG=en_US.UTF-8
LC_TIME=en_DK.UTF-8

• Verify that printformats picks it up:

khal printformats                         
longdatetimeformat: 2013-12-21T21:45:00 
datetimeformat: 2013-12-21T21:45:00 
longdateformat: 2013-12-21
dateformat: 2013-12-21
timeformat: 21:45:00

• Look for events at a date:

khal at 2026-05-14T12:00:00:00             
critical: Could not parse "['2026-05-14T00:00:00']".
critical: Please check your configuration or run `khal printformats` to see if this does match your configured [long](date|time|datetime)format.
critical: If you suspect a bug, please file an issue at https://github.com/pimutils/khal/issues/ 

• Edit your configuration to manually define that same datetime format:

[locale]
local_timezone= Europe/Paris
default_timezone= Europe/Paris
timeformat= %H:%M:%S
dateformat= %Y-%m-%d
longdateformat= %d.%m.%Y
datetimeformat= %Y-%m-%dT%H:%M:%S
longdatetimeformat= %Y-%m-%dT%H:%M:%S

• Run khal with the specified format

khal at 2026-05-14T12:00:00
2026-05-14T12:00:00
2026-05-14T12:00:00-2026-05-14T13:00:00 [REDACTED for privacy]

Expected behavior
I expected khal to parse the format in khal at as specified in khal printformats or khal printformats to return an error about the format not being recognized and requesting manually setting it in the configuration.

OS, version, khal version and how you installed it:

• khal: version 0.14.0
• OS: Archlinux
• Installation method: OS repo: extra/khal
• Python: 3.14.4
• Khal config file causing the issue:

[calendars]
  [[default]]
    path = ~/Calendars/default
    color = dark magenta
    priority = 20

  [[reminders]]
    path = ~/Calendars/reminders
    color = dark green

Start by reproducing the issue with the locale.conf and khal configuration shown, then compare the parsing used by khal at with the formats reported by khal printformats. Trace the date and datetime parsing entry points and add a regression test for the ISO8601 system-default case. Done means the command accepts the displayed format without manually duplicating it in the configuration.