Here is how to figure out critical hours:
Locate your AM station license (hopefully you do have a copy of it readily available). You will need the geographic coordinates of the tower site, which will be on that license.
You can go to the
FCC sunrise/sunset time calculator and determine the monthly average sunrise and sunset times for your station location. Then your critical hours times will be two hours
after local sunrise, and two hours
before local sunset. Remember that these times are different for each calendar month.
If you have an older-style paper copy of your AM station license (given the specifics of your station, that is quite possible), the sunrise-sunset times are probably already spelled out on the existing license.