Go to the 'Examination Dates and Reservations' section and make sure that you have selected the right term (see the top of the page). Click on the appropriate code of course, which will take you to the page listing its examination dates. Unless it is 17:00 yet, the dates are red, i.e. you still cannot reserve any slot on them. Each of the dates contains the 'attempt to reserve a slot' reference. Provided you click on one of the references before 17:00, you will get an error message. Nevertheless, you can use the Refresh/Reload button of your browser to repeat the operation until you succeed in the reservation.
You can perform this operation any time before the start of reservation period. You can even open several windows, load them with different examination dates and keep refreshing their contents switching between these.
Please bear in mind that the workload of the System might increase dramatically shortly before 17:00 and thus you might not find enough time for the preparation. It is therefore advisable to begin earlier.

I want to find out what the current IS time is because I want to be ready for reservations at exactly 17:00.

If you want to check your ranking in the time competition, you can use the application "Course and Seminar Group Registration Position".