Date compare in scheduling emplooyee job

Because, in my office, sometimes the job admin forget that a worker is already in the job. Example is Mr. John already in job in Sept 5-sept 8, because the job admin forget, he will give another job to mr John in sept 7-8,,,so the usage of this website sistem is to prevent that happens, so when in happen, the detail will show that Mr.Jhon is already in job  from 5 Sept to  8 Sept,,,please choose another employee to do this job.