How can we use UTC, as to be very accurate, we don't know what the exact UTC time was until after it has happened.

Atomic and Solar Time
Two components are used to determine UTC:

International Atomic Time (TAI): A time scale that combines the output of some 200 highly precise atomic clocks worldwide, and provides the exact speed for our clocks to tick.
Universal Time (UT1), also known as astronomical time or solar time, refers to the Earth's rotation. It is used to compare the pace provided by TAI with the actual length of a day on Earth.

Universal Time (UT1), also known as astronomical time or solar time, refers to the Earth's rotation. It is used to compare the pace provided by TAI with the actual length of a day on Earth.

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A problem with this is that earths rotation is not constant.
It could differ a few seconds from one year to another.
Is this a problem?
Yes for GPS systems.
Not only that. The GPS satellites are affected by less gravity then here at earth. That means their clocks are ticking faster than ours.
This is syncronized by many stations here on earth.