Ancient cultures had no problem determining that the year was a little bit longer than 365 days; anyone with curiosity and patience can do the same now by watching the sun rise (or set) from a fixed spot every day, marking the spot over which it rises/sets, and continuing until the sun, crossing a particular spot and moving in the same direction as it did the first time, returns to the same spot. The number of days elapsed will be one year.

Much of the variety of actual calendars, however, grew out of attempts to overcome two major difficulties: 1) the 365+a-little-bit was not as mathematically-friendly number as the nearby 360 (which is easily divisible by 2, 3, 4, 5, 6, 8, 9, 10, 12 (good for months), 15, 18, 30, 36, 40, 45, 60, 72, 90, 120 and 180), and 2) the lunar calendar, which had the advantage of parts of months being easily and immediately recognizable by the visual shape of the moon, could not be coerced into either 365+a-little-bit or 360 day years.

The point is that the length of the solar year in Moses' day was indistinguishable from our own reckoning, so the Genesis ages should be understood in terms of years as we understand them.

I do remember, but without access to links, that a number of creation scientists posit a natural cause that may have been employed by the Lord to rapidly reduce human ages between the pre- and post-Flood generations. They believe there would have been a huge increase in solar radiation reaching the earth's surface after much of the water vapor in the pre-Flood atmosphere fell to earth, resulting in far more rapid aging than previously.

In Christ,
Paul S