How do I time exclusions and signal re-entry?

Under World Aquatics rules (Part Six, Articles 3.10, 3.11 and 10):

Ordinary exclusions (18 seconds)

• The timekeepers record each exclusion and the re-entry time of the player or substitute.
• An excluded player may re-enter at the earliest of: 18 seconds of actual play; a goal; their team regaining possession (the Referee signals re-entry by hand); or their team being awarded a free throw, goal throw or penalty throw.
• When the exclusion expires on time (rather than ending through a change of possession), the secretary signals it by raising the appropriate flag (or another approved device).

Violent action (4 minutes)

• The excluded player is out for the rest of the match, and a substitute may enter only after four minutes of actual play.
• The secretary signals that the four minutes are up by raising the yellow flag together with the team's white or blue flag (matching its cap colour), or another approved signal.

Simultaneous exclusions: the Referees tell the secretaries which players are excluded, and all of them may return at the earliest of 18 seconds, a goal, or the next change of possession.

The actual game clock keeps running only during actual play, so the 18 seconds and 4 minutes count actual play time only.