In hook_install() of your module define the appropriate array (name, label, pattern, module, hidden) and add it to default date settings with: system_date_format_save($date_format)
Then add hook_uninstall() to remove the formats when the module is uninstalled.
Wow, that sounds awful...
So maybe it's not even related to other modules, but some... cache problem somewhere? Be it browser or server-side? A Heisenbug.
Comments
Never mind, I figured it out.
In hook_install() of your module define the appropriate array (name, label, pattern, module, hidden) and add it to default date settings with: system_date_format_save($date_format)
Then add hook_uninstall() to remove the formats when the module is uninstalled.
I guess, my question is answered. ;)