Tax deduction u/s.192 is warranted only at the time of actual payment of salary.
Therefore, in case where an employer follows mercantile system of accounting and provides salary on accrual basis, tax shall not be deducted u/s.192.
It may be noted that u/s15, salary income of an employee shall be chargeable to tax on due or receipt basis, whichever is earlier.
Still, for the purposes of deduction of tax u/s.192, it is always to be made on actual payment of salary irrespective of the method of accounting adopted by the employer.
As far as bonus, it is to be made on payment basis.If bonus is provided in the books of accounts for the month of march, then such provision is allowable only if it is paid within 30 days immediately.