I don’t know the history, but I want to point out that dawn isn’t standard.

The sun rises at a different time every day, gradually happening earlier and earlier from the winter solstice to the summer solstice, and then later and later as we continue back to the winter solstice.

Sunrise also happens at different times in different places. We have timezones to standardize time-keeping, but the sun rises earlier on the eastern side of a timezone than on the western side. Sunrise timing is also affected by latitude, because the earth’s axis is tilted relative to its plane of orbit. If you go far enough towards the poles, you’ll reach places where there is no sunrise for months in both the summer (the sun doesn’t set so it also doesn’t rise) and the winter (the sun is absent entirely).

So, dawn isn’t particularly well-suited to precise record-keeping, especially for records that need to be shared or synchronized between people in different geographic locations.