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This time is from the most accurate clock in the world - a stratum-1 atomic clock operated by NIST. Alas, due to delays in the internet, this page won't display fast enough for the time to be exactly right. But it should only be a few seconds off.
The format is "yy-mm-dd hh:mm:ss" Greenwich Mean Time (GMT) - "standard" time in England (not daylight-savings, or summer time). Calling it GMT is actually somewhat antiquated. It's more properly called UTC (Coordinated Universal Time). Same thing, different name.
For more info on the time server, see the NIST Internet Time Service page.