Hi Mike,
Great idea. Adding some code like this (to your theme’s functions.php file or some other suitable place) would be one way to change the default start and end time.
It hasn’t been extensively tested by any means (and doesn’t support more advanced ideas such as setting the end date one day ahead etc) but should certainly give you a starting point – two example uses are given at the bottom of the snippet.
If that doesn’t work for you – or even in addition to using it – you could also consider posting a feature request over on our UserVoice page at tribe.uservoice.com … thanks!