I have read http://www.yiiframework.com/wiki/24/creating-a-dependent-dropdown/ (Dependen Dropdown Yii1), but I can't implement it in Yii2 because Yii2 does not have built-in AJAX functionality hem.. I searched about it and came across this post
Following on from international dates, I also wanted times in the local timezone and format. This was a little more tricky but I think I've got a solution. This is only really appropriate if you have an international app.