1. choosing the date format specified. (Dashboard → Settings → Formatting → Date Header Format) and usually the correct type is: Saturday, 5 May, 2009.
2. For the UNDEFINED (date and time) part, this is what you should do:
Go to LAYOUT -> PAGE ELEMENTS -> BLOG POSTS (edit) and then:
On SELECT ITEMS’s FIRST BOX choose the third option (the one with the DAY,MONTH,YEAR), on the SECOND BOX write the MONTH and the YEAR and on the THIRD BOX select the FOURTH OPTION (the one with the DAY, MONTH, YEAR).
Timestamp Format → Friday, May 7, 2010.