You might want to check out PHP's documentation, as these are basic date manipulation functions: http://php.net/date

Also, all entities in Elgg are stored using an epoch time stamp as $entity->time_created

As for CalendarPopup.js, that is quite old and should be replaced by something from jQuery UI.  Mind adding that to trac?

Indeed replace Calendar would solve as it can prevent users to just enter everything they like for a date. There are several jquery calendars, but we intend to use ui.datepicker. Unless someb will suggest us different , we will try to implement and post result, including on Trac to assist later official update.

ref: http://docs.jquery.com/UI/API/1.7.2/Datepicker and http://jqueryui.com/demos/datepicker/

Tom, you need to check out my plugin: custom profile fields. This plugin adopts the datepicker and supports locale and language specific date formats

Would be useful to have a datepicker plugin that supports Elgg 1.5 as well (ie. jQuery 1.2.x)

The datepicker i use does that. I think it is the same that you use Kevin in the eventcalendar. It is the jquery datepicker that formed the basis for the one mentioned by Tom, but has a lot more capabilities.

@Brett: done http://trac.elgg.org/elgg/ticket/1297

@Jeroen; yes, but we needed stronger sanitize of existing fields and uniform language solution; could not use the locale in multiple language sites

ALL: We tried to solve with http://community.elgg.org/pg/plugins/tomv/read/292359/datepicker   :)

@Kevin and @Jeroen: are you planning to modify event_calendar and custom_profile_field to extend Tom's datepicker plugin?