There is already a function for deleting old logs.
Often you are setting up a website and include piwik before going live and /or you are doing some test with piwik.
when going live with your website or after the tests are done,
it would be very usefull to extend this function to simply delete "all logged things" before a special date+ time
two field and one button may be used:
field 1: to put the date in
field 2: to put the time in
a button: "delete every thing before"
showing in addition the "actual piwik date and time" directly at this place, may be avoid problems if there is a differnt/ mis-setup server/piwik time...