What does "Timestamp" mean?

http://euw.leagueoflegends.com/en/page/re/play?utm_source=LeagueClient&utm_medium=OverviewTier2Pos3 So we can now submit replays of {{champion:497}} {{champion:498}} to riot and we need to say on the subject: “Summoner Name - Country Of Origin - Champion Name - Timestamp.” What tha hell is a "Timestamp"? I don't know what to say there. Plz help and sorry if im being a noob
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