Help with time (shift) tracking
 

I do payroll for clients that are not permitted to have more than one
employee working for them at the same time during any given period. I am
trying to come up with a way to enter an employee's shift (ie 9am-5pm) and
then upon entering another employees shift (4pm-10pm) present an "error"
message of some sort if there were any overlapping in hours (in this case,
the overlap would be from 4pm-5pm). I have 150 employees and 100 clients, so
our current method of color-coding each hour worked is extremely
time-consuming. Can anyone please help me with some ideas? Thanks!