How to center date header and change font size

"Do you know how centre the date?????"

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

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

and if you also want to change the font size, add this line - font-size:200% or font-size:12px; or whatever values you wish, so that the codes become

/* Posts
-----------------------------------------------
*/
h2.date-header {
margin:1.5em 0 .5em;
text-align:center;
font-size:200%;
}