According to the NHRA rule book, the end date is predetermined
From the 2007 NHRA rulebook:
10:7 HELMETS
As outlined under Class Requirements, drivers in all classes must
wear a helmet meeting Snell or SFI Specifications. Snell K98 is
acceptable in place of any Snell M-rated helmet.
SFI Spec 31.1A = Snell SA, open-face helmet
SFI Spec 31.2A = Snell SA, full-face helmet
SFI Spec 41.1A = Snell M, open-face helmet
SFI Spec 41.2A = Snell M, full-face helmet
Drivers of all cars running 13.99 or quicker must use a helmet
meeting Snell K98, 2000, 2005 or SFI Spec 31.1A, 31.2A,
31.1/2005, 41.1A, 41.2A, or 41.1/2005.
Drivers of cars running 7.49 or quicker must use a helmet meeting
Snell SA2000, SA2005 or SFI 31.1A, 31.2A, or 31.1/2005 Specs.
See Class Requirements.NHRA Helmet Expiration Dates
98 SECTION 11 - GENERAL REGULATIONS GENERAL REGULATIONS
Label Expires
Snell 90 Expired
Snell 95 Expired
Snell K98 1/1/2009
Snell 2000 1/1/2012
Snell 2005 1/1/2017