They had nothing to compare the clocks against, and I don’t think the stars and the sun would have made an useful target for comparison.
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The stars are still used as a comparison target. They were the primary time reference up until the 1950s or 1960s.
Any given star will pass through the meridian of any location 366.2422 times per year, at an extraordinary degree of consistency. A star is directly overhead at midnight last night? It will be overhead at 11:56:04.091 pm tonight. A different star will be directly overhead at midnight. Given a catalog of star locations (which predate clocks by over a millennium), accurate sky observation tools (ditto), and a calendar (ditto again), and you can tell the time with very good precision (on clear cloudless nights).
In the 20th century we finally made clocks that were more consistent than the rotation of the Earth, and we stopped using the stars as the primary source of time.
The Sun is not as good for the most accurate timekeeping. The time between solar noon one day and solar noon the next varies by several minutes over the course of a year, and is only 24 hours on average.
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