In this section, “chief executive officer” means: the mayor of Baltimore City; or the County Executive of Montgomery County. This section applies only to Baltimore City and Montgomery County. Except as provided in subsection (d) of this section, a candidate for chief executive officer shall be a resident of the county for at least 6 months before the general election for chief executive officer. If the charter of a county contains a durational residency requirement that is longer than 6 months, the longer durational residency requirement shall be retained if the governing body of the county reaffirms the requirement by enactment of an ordinance no later than 4 weeks before the filing deadline specified in § 5–303(a) of the Election Law Article.