How does a period end, and what if a penalty or shot is happening at the buzzer?

Under World Aquatics rules (Part Six, Articles 3.10.5, 7.7 and 16.8), the timekeeper signals the end of each period by whistle (or another audible signal), and the signal ends the period immediately, except:

• Penalty awarded at the same moment: the penalty throw is taken first. All players except the shooter and the defending goalkeeper leave the field into the flying substitution area. The period ends once the throw has been taken (scored, missed, or rebounded).
• Ball in flight: if a shot is in the air when time expires and crosses the goal line without anyone else gaining possession (and without an attacker deliberately playing it), the goal counts, including after bouncing off the water, the goalkeeper or a defender, or an unintentional touch by an attacker. Otherwise the period ends.

The timekeeper also announces the start of the last minute of the match.