org mode in emacs has this wonderful feature of integrating the calendar in itself. A C-c . key binding in org mode invokes the calendar. you can move around dates with the S- ->(shift right arrow: forward day) and S-<-(shift left arrow: backward day). but this is only in an org-mode buffer, buffers which have a .org extension or buffers where you manually switch on org-mode by M-x org-mode RET.
one can enable this on all types of buffers globally. put this in to your .emacs
(global-set-key “\C-c.” ‘org-time-stamp)
so in any buffer, one can C-c . to get in a date time stamp.