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Hello,
The main problem was that some module use the old date format for split and check if the date is ok and have a correct format.
So they check Y-m-d and wait for a date like 2012-02-22 and return an error (see extcal)
This problem is not a core error, but a module error and can't be fixed by core change.
I have include a CAL_FORMAT for adapt date in each site.
Actually, date is set to Y-m-d for a legacy use, like old version (eg. all old version use only this date format), but if a site use a fresh module that don't check the date and use the simple way for save it, you can use an other format d/m/Y or an other.
Any other way, broke old module, and if we include an easy way for date, we must do it in 2.6.
Nicolas