Severance pay is only deductible at the point of payment for calculating taxable income. Even if employment contracts or other company policy states a formula for severance pay making accruals possible, the timing a company can claim as tax deductible is at the time of payment. Accrued expense in general is accepted, but timing for payroll payment is predominately cash basis.
Of course you can accrual severance pay for your financial statements, but be sure not to forget to make adjustments when adjusting taxable income. In the corporate tax return, there is a document which will show the adjusting figures up to date. Confirming balance outstanding every year for historical adjustment is advised.