How to change alignment of the date header

Open the template editor by going to Dashboard > Layout > Edit HTML and look for these codes:

/* Posts
-----------------------------------------------
*/
h2.date-header {
margin:1.5em 0 .5em;
}

and change it to whatever alignment you want

/* Posts
-----------------------------------------------
*/
h2.date-header {
margin:1.5em 0 .5em;
text-align:center;
}