Just copy and paste the following lines into the pdt you wish to modify for variable visibility.
The movement and combat modifier have been set so they ahve no affect on the game.
Place these lines right after the Night Movement Fatigue line in the pdt which should be 16 lines after
the last weapon in the list.
Make sure you adjust the date and times to match the game you are playing.
This list is set up for a 400 dawn and a 2000 (8 PM) night turn so adjust the times to match the
time of year for your scenario.  If you like you can lengthen the period before full visibility 
by adding more lines.
For a multiple day battle make sure to duplicate the lines below for the additional days
changing the date as needed.

1 1863 6 28 4 0 100 2 100 100 0 0 Twilight
1 1863 6 28 4 20 100 3 100 100 0 0 Near Dawn
1 1863 6 28 4 40 100 4 100 100 0 0 Dawn
1 1863 6 28 5 0 100 8 100 100 0 0 Sunrise
1 1863 6 28 5 20 100 12 100 100 0 0 Morning
1 1863 6 28 5 40 100 20 100 100 0 0 Hazy
1 1863 6 28 6 0 100 40 100 100 0 0 Clearing
1 1863 6 28 6 20 100 70 100 100 0 0 Clear
1 1863 6 28 17 40 100 40 100 100 0 0 Smoky
1 1863 6 28 18 00 100 20 100 100 0 0 Hazy
1 1863 6 28 18 20 100 12 100 100 0 0 Setting Sun
1 1863 6 28 18 40 100 8 100 100 0 0 Sunset
1 1863 6 28 19 00 100 4 100 100 0 0 Dusk
1 1863 6 28 19 20 100 3 100 100 0 0 Near Twilight
1 1863 6 28 19 40 100 2 100 100 0 0 Twilight
0