Detecting gravitational waves

Scientists have built a gravitational wave detector, with detector stations all around the world. Unfortunately, the gravitational waves are extremely faint, and often, what the sensors pick up is just noise. Scientists say that a recording can only be trusted if four or more detector stations around the world are triggered at precisely the same time. Only then can we say with a reasonable degree of certainty that we've detected a gravitational wave¹.

In the input list, you find the local times when detectors recorded something, and the time zone of that detector station. Go through the list, and find an instance where four signals were recorded at the same time (at least within the same minute).

For example, in the list of six timestamps below (your test input):

2019-06-05T08:15:00-04:00
2019-06-05T14:15:00+02:00
2019-06-05T17:45:00+05:30
2019-06-05T05:15:00-07:00
2011-02-01T09:15:00-03:00
2011-02-01T09:15:00-05:00

In this list, the first four recordings took place at the same time, in different time zones. 08:15 in time zone GMT-04:00 (Toronto summer time) happened at the same time as 14:15 in time zone GMT+02:00 (Paris summer time). The last two recordings were both made at 09:15, but they actually differ by two hours.

Your puzzle input is a complete list of recordings. Find the time that a gravitational wave was recorded in four or more places at the same time. Your final answer must be in the exact same format, but with the time zone offset normalized to '+00:00'. In the example above, the answer corresponding to the four contemporary recordings would be 2019-06-05T12:15:00+00:00

When was a gravitational wave recorded in four or more places, expressed as a timestamp normalized to '+00:00'?

Reading & reference materials

• UTC on Wikipedia, a time standard
• UTC offset, how the same moment in time has different local readings around the globe

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