I checked the code a bid and found this line in the html code

<style type="text/css">@import url(/css/calendar-blue.css);</style>

Somehow it is ignored or the leading / is the prpoblem. I didnt find the part in the php code where this line come from:(

A little workarround is to add the css file in the config_inc.php.

I simply importeded the css file in my separate metatgafile which I load with $g_meta_include_file

example:
$g_meta_include_file = "css/metatags.txt";

metatags.txt:
<link rel="stylesheet" type="text/css" href="css/calendar-blue.css" />

And a little question:
Why is this not added for custom fields of type date?

fixed in master-1.2.x and master