#Value in random cells

I have a formula to calculate the hours I work based upon time in and time
out - time in is deducted from time out. Since my time is based on two weeks
and starts on Sunday, ending on Saturday, my formula uses the ISBLANK
function, as I don't always have to come in on weekends. However, I have
random #VALUE! errors in the cells that show the time worked. Most of the
cells show a blank if there is no start time for the day, but I currently
have the value error on Monday and Tuesday of the second week. No matter how
many times I copy/paste from a working cell, or manually type in the formula,
this error won't go away. Any ideas as to what I might be missing? Thank
you.