Icd-10

For the purpose of billing, if I have encounters with dates of service say August and of Sept. 30, 2015 would I be using ICD10 based on date of service or when the encounter was billed out? I listened in on a class for ICD-10 and they said we would have to bill as ICD10 if we billed out after Oct. 1,2015 even if encounter was prior to OCT.
Could you please clarify.
Thank you,
Nilsa

Icd-10