Set default date in datepicker of an advanced custom field

It depends on the datepicker plugin you are using. Mostly all datepickers will have a default value you can play with. Usualy it is set within the input text type that hosts the datepicker.

<input type="text" class="datepicker" default-value="2013/07/25">

And in wordpress’ php you can do :

<input type="text" class="datepicker" default-value="<?php echo date("Y/m/d"); ?>">