Some themes have time stamps and some do not. This is the choice of their designers. Changing that in a theme can be requested in the ideas forum but be prepared for a very long wait.
Meanwhile if a timestamp is an essential feature to you then there is a list of themes that do and don’t have them in this thread http://wordpress.com/forums/topic.php?id=1703&replies=6#post-19934
And if changing themes is not satisfactory to you and you have the expertise to undertake css customization of the Regulus theme then you can purchase an upgrade.