The Secretary of State may from time to time establish a list of places which by reason of climatic or other extreme conditions are to be classed as unhealthful posts. Each year of duty at such posts, inclusive of regular leaves of absence, shall be counted as one and a half years in computing the length of the service of a participant for the purpose of retirement, fractional months being considered as full months in computing such service. No such extra credit for service at such unhealthful posts shall be credited to any participant who is paid a differential under section 5925 or 5928 of title 5 for such service. Such extra credit may not be used to determine the eligibility of a person to qualify as a former spouse under this part, or to compute the pro rata share under section 4044(10) of this title. No extra credit for service at unhealthful posts may be given under this section for any service as part of a tour of duty, or extension thereof, beginning on or after February 16, 1990.