(a) In counties of the second class, the precepts to make assessments in the years between triennial assessments shall be issued to the appointed assessors by the board for the assessment and revision of taxes at such time as the board may prescribe, and return thereof made on or before the first Monday of November as provided by existing law relating to the board for the assessment and revision of taxes in said counties.

(b) In counties of the third class, the precepts to make assessments in the years between triennial assessments shall be issued to the appointed assessors by the board for the assessment and revision of taxes, and return thereof made at such times as the board shall determine in accordance with existing law relating to the board for the assessment and revision of taxes in said counties.

(c) In counties of the fourth class, the precepts to make assessments in the years between triennial assessments shall be issued to the assessor by the county commissioners on or before the first day of June, and the assessors are hereby required to complete the said assessments and to make return thereof not later than ninety days from the date of the issuing of said precepts.

(d) In counties of the fifth, sixth, seventh and eighth classes, the precepts to make assessments in the years between triennial assessments shall be issued to the assessors of townships of the second class, towns, boroughs and wards on or before the second Monday of September, and, to assessors in townships of the first class, on or before the first day of July, and the said assessors are hereby required to complete the said assessments and to make return thereof not later than ninety days from the date of the issuing of said precepts: Provided, That in cases of an emergency, or in wards where an assessor cannot complete the reassessment within the said ninety days, the county commissioners may, at their discretion extend the time for completing the said reassessment and for making return thereof: And provided further, That where assessors in townships of the first class shall have been continuously engaged in the actual performance of their duties after the delivery of the precepts to them, except where prevented by sickness or stress of weather, and are not able to complete the assessment and make return thereof as herein provided, it shall be lawful for said assessors to continue the performance of their duties and to make return of their assessment to the county commissioners after said date, but in no case shall any such return be made later than the fifteenth day of February of the year following the delivery of the precepts to the assessors.