For a registered assessee who is paying service tax only on reverse charge basis, he is not entitled to any CENVAT Credit. But if the liability is also discharged as a service provider then they can adjust the CENVAT Credit on the amount of ST paid on reverse charge basis, provided that the said service is not used exclusively for an exempt service or an exempt product.
Service tax liability on reverse charge basis is on the date on which the payment is being made to the service provider.
But for a service provider, the point of tax is the date of invoice or the date of payment received whichever is earlier. Hope it answers all your queries.