How long is a match and who keeps time?

Under Law 5:

• A match lasts no longer than 80 minutes, split into two halves of not more than 40 minutes plus time lost, unless the organiser has authorised extra-time in a drawn knock-out match.
• Half-time is an interval not exceeding 15 minutes, as decided by the match organiser.
• In non-international matches the organiser may shorten the match. If the organiser does not decide, the teams agree, and if they cannot agree you decide. (The Game-On variation says games should last a minimum of 40 minutes and can be split into equal halves, quarters or thirds.)
• You keep the time, but may delegate it to either or both assistant referees or an official time-keeper, in which case you signal any stoppage to them. If there is no official time-keeper and you are in doubt, you consult either or both assistant referees.
• You may stop play and allow time for a player injury of up to one minute (more at your discretion if a player is seriously injured), and for consultation with other officials.