How is the official time worked out when timekeepers' watches are used?

When no automatic or semi-automatic equipment is used, World Aquatics Rule 11.2 applies:

• Each athlete's time is recorded to 1/100 of a second by two timekeepers.
• If the two watches differ, the official time is the average of the two. If the average runs to thousandths, the last digit is dropped, not rounded (for example, 31.24 and 31.27 average 31.255, recorded as 31.25).
• If one watch fails, the time on the working watch is the official time.
• If both watches fail, the swimmer may be offered a reswim.
• The referee decides the winner and placings from the times; swimmers with equal times are placed equal.
• Each timekeeper must be appointed or approved by the national federation and use a watch certified as accurate.

Even when automatic equipment is in use, it may be necessary to have the full complement of timekeepers as back-up unless video timing is used.