26 C.F.R. § 41.4483-4
Any exemption from the tax on the use of a highway motor vehicle has application only with respect to the use of such highway motor vehicle and not with respect to the highway motor vehicle as such. Furthermore, such exemption is subject to those provisions of paragraph (c) of § 41.4481-1 relating to proration of the tax and to the effect of an exempt use of a highway motor vehicle after a taxable use has been made. Thus, if a taxable use is made of a highway motor vehicle at any time in a taxable period, the tax is imposed on the use of such vehicle for such taxable period, computed from the first day of the month in which such taxable use occurred, even though at some time in the same taxable period, before or after such taxable use occurred, the use of the vehicle may have been, or may be, exempt. For example, if a highway motor vehicle is operated exclusively by a State in the period July 1 through September 10 of a taxable period, use of such vehicle in such period is exempt from the tax. However, if a taxable use of the vehicle is made on September 11 of such taxable period, the tax imposed on the use of such vehicle for such taxable period is computed from September 1. On the other hand, if a taxable use of the vehicle is made at any time in July of the taxable period, the tax imposed on the use of such vehicle for such taxable period is computed from July 1, even though the vehicle may be operated exclusively by a State in every other month of such period.
26 C.F.R. §41.4483-4