Sec. 6. (a) The effective date of a modification of the agreement must be determined by resolution of the board of each of the following retirement systems:
(1) The applicable pension system administered by the Indiana public retirement system.
(2) Any retirement system established by a state educational institution.
For political subdivisions the governing body shall determine the effective date by resolution. The effective date may be made retroactive to the extent permitted by federal law.
(b) The effective date of a modification for employees of political subdivisions with retirement systems which are not covered by subsection (a) may be January 1, 1955, or any subsequent January 1.