I have no doubt that your internal memo said that, I also have no doubt that it is not actually the law and that the memo is wrong (or that you misinterpreted it). When you are working off hitting the 180 school days, then each day must be at least 6 hours to be counted. (180x6=1080) The new law allows more flexibility though. The new law clearly says 180 days or 1080 hours. Yes, you can lengthen the school day and reduce the number of days or you can do like this particular school in ALABAMA did, and have some half days but the other days longer than 6 hours so that cumulatively the amount comes up to at least 1080 hours:
http://data.madisoncity.k12.al.us/Documents/Calendars/MCS 2013-2014 Calendar- Approved.pdf
Feel free to count the days in school. There are 176 full days and 4 half days. I can go through other Alabama school district and show you the same thing. You may feel I'm picking on you, but I'm not. Often people in positions of authority claim things that aren't true. In some cases it is simply misinterpreting the rules/laws, in other cases it is more willful. Perhaps whoever is in charge in your area believes it to be true, or perhaps he/she feels that half days are unproductive and feels it is easier to just say half days can't count. Either way, it is wrong.
My school district tried to tell be I needed to register my kids as home schooling students. They were quite insistent that it was the law and sent us very official looking memos telling me what I had to do. The problem was that the law said no such thing. I do not need to register my kids as home schoolers nor do I need to inform the local district of my plans. I sent a letter to the district with a copy of the relevant law plus all the NJ Supreme court decisions regarding home schooling and I never heard from them again. Their website still claims that home schoolers need to register. Obviously they are hoping that most people will just comply without question.