Have a look at the getHours() and getMinutes functions:
var hour:Date = new Date();
var minute:Date = new Date();
hours = hour.getHours();
minutes = minute.getMinutes();
This code sets up two variables, then usesthe functions to return string values for the current hours and minutes. The code above uses local PC time, though there is a function (check the help files) that gets a standard internet time if that's important.
Once Flash has the time, then a simple 'less than' check will allow you to change movieclip.