Often, to identify a future filing deadline, a litigator must identify the date of service of some document filed previously by opposing counsel. In practice, the date of service is often indicated by the date written in a signed “Certificate of Service” attached to the document. Certificates of Service are not mandatory and failure to attach one does not affect the validity of service. NRCP 5(b) (4). Nonetheless, filings typically include them as proof of service.