The problem with each subject having a designated time period is that the students STARE at the board ready to tell me, "It's time for math".
"Ummm...it's time for math when I say it's time for math. We need to finish our writing first". This was a common scenario for years.
Finally, I learned to just write the subjects in order of which I was teaching them, and not assigning a specific time period.
A small pocket chart was a wonderful solution. I use sentence strips to indicate the order in which our lessons and activities would take place. I keep the lunch and playground in the chart as a constant. A small envelope, attached to the white board with a clip magnet, contains the strips that would be placed in the chart each morning before the students arrive. The envelope contains strips for days of our school cycle, subject areas and special classes.
The students literally use this chart as their basis for the whole day. Once I forgot to change the strips and each student walked in saying, "Wow we have art again today!". I had to laugh at how they take whatever that chart reads as fact! Ahhh...kids, gotta love them!