Using the shortcode to override how start timers and end timers effect the forms:
If you use showform=1 the form is shown and entries are accepted regardless of the start and end timers being set or not.
If you use showform=0 the form is hidden and entries are NOT accepted regardless of the start and end timers being set or not.
Default handling without using the showform short code:
If you do not set a start time, then the form is shown while voting is taking place.
If you set a start time for the future, the form is shown until the voting starts and then the form is hidden and entries are closed/not accepted.
If you set a start time for the past, the form will not show up and entries are closed/not accepted.
The end timer, whether set or not, does not effect if the form is shown or hidden unless the contest has ended. Once the contest ends, the entry form will not be visible.