It is 100% legal. An employer is free to look at, analyze, compare, etc. employee timesheets: the employee has no reasonable or legally enforceable expectation of privacy in their timesheets against their own employer, and the timesheets may be about the employees but are the property of the employer--and it can use its property however it likes. Moreover, the employer doesn't even need to compare the time sheets: unless an employee has a written employment contract, he or she is an "employee at will" and the employer could discipline, even fire, the employee based on nothing more than a belief or suspicion.