Time Calcualtions and #VALUE

Hi, I am trying to put together a simple weekly timecard of sorts to help me track my hours/utilization on a 24hour work schedule. I have the basics of the timecard compiled, however, when time is entered on a day, and then removed for whatever reason, I recieve the #VALUE error. I would like to be able to add or remove time as needed, and have the cells remain blank when not used. I have attached an example of one week. I am trying to keep it rather simple and any help is much appreciated.
Thanks!