(a) General rule.--Except as set forth in subsection (b), the term "prior offense" as used in this chapter shall mean any conviction for which judgment of sentence has been imposed, adjudication of delinquency or other form of preliminary disposition before the sentencing on the present violation for any of the following:
(1) an offense under section 3802 (relating to driving under influence of alcohol or controlled substance);
(2) an offense under former section 3731;
(3) an offense substantially similar to an offense under paragraph (1) or (2) in another jurisdiction; or
(4) any combination of the offenses set forth in paragraph (1), (2) or (3).
(b) Timing.--
(1) For purposes of sections 1553(d.2) (relating to occupational limited license), 1556 (relating to ignition interlock limited license), 3803 (relating to grading), 3804 (relating to penalties) and 3805 (relating to ignition interlock), the prior offense must have occurred:
(i) within 10 years prior to the date of the offense for which the defendant is being sentenced; or
(ii) on or after the date of the offense for which the defendant is being sentenced.
(2) The court shall calculate the number of prior offenses, if any, at the time of sentencing.
(3) If the defendant is sentenced for two or more offenses in the same day, the offenses shall be considered prior offenses within the meaning of this subsection.
(4) When the completion date of the Accelerated Rehabilitative Disposition program or a pretrial diversion program substantially similar to Accelerated Rehabilitative Disposition in another jurisdiction for an offense under section 3802 or a substantially similar offense in another jurisdiction is more than 10 years prior to the date of the offense for which the defendant is being sentenced, a prior conviction for violating section 3802(h)(1) shall be treated as a prior offense for the underlying violation of section 3802.