Calendar system requires a very tiny core patch to fully work, The patch was already proposed to core by a fellow developer, so i rewrote it to meet core requirements and set it to needs review (Thanks to Gaelan and wuinfo for helping in completing the patch). The good thing is that the patch is not only for Calendar Systems it's a generic patch that introduces a new hook to make it possible for third-party modules to alter format_date function. If we can get it into core it might even be possible to back port it to Drupal 7

You can read more about the history of calendar system module and even what a calendar system is

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