In all wp.com themes, posts display the publication date – goes along with the very idea of a blog. Static pages, on the other hand, sit outside the chronological structure of a blog; so as a rule they don’t display the publication date.
There are some exceptions, though:
• In Andrea, Chaos Theory, Duotone, Fleur de Lys, Garland, Girl in Green, Green Marinée, Iceburgg, Jentri, Monotone, Next Saturday, Rounded, Spring Loaded, and Toni, the date shows up.
• In Monochrome the default is no date, put you can change it if you wish (page editor > “Attributes” module > “Template” dropdown).