Dear Ashish,
The above Notification No.18/2011 has been amended vide new Notification No.25/2011 dated 31.03.2011, which provides for deposit of Servicetax on Payment or receipt basis as the case may be, in respect of assessees paying service tax as per rule 2 (1) (d). So you need not pay Service Tax on Billing basis. The relevant part of the notification and the link is given below for your reference:
Link:https://servicetax.gov.in/st-notfns-home.htm
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5. For rule 7, the following rule shall be substituted, namely:-
“7. Determination of point of taxation in case of specified services or persons.-Notwithstanding anything contained in these rules, the point of taxation in respect of,-
(a) the services covered by sub-rule (1) of rule 3 of Export of Services Rules, 2005;
(b) the persons required to pay tax as recipients under the rules made in this regard in respect of services notified under sub-section (2) of section 68 of the Finance Act, 1994;
(c) individuals or proprietary firms or partnership firms providing taxable services referred to in sub-clauses (p), (q), (s), (t), (u), (za), (zzzzm) of clause (105) of section 65 of the Finance Act, 1994,
shall be the date on which payment is received or made, as the case may be:
Provided that in case of services referred to in clause (a), where payment is not received within the period specified by the Reserve Bank of India, the point of taxation shall be determined, as if this rule does not exist.
Provided further that in case of services referred to in clause (b) where the payment is not made within a period of six months of the date of invoice, the point of taxation shall be determined as if this rule does not exist.
Provided also that in case of “associated enterprises”, where the person providing the service is located outside India, the point of taxation shall be the date of credit in the books of account of the person receiving the service or date of making the payment whichever is earlier."