Indiana Code Title 5. State and Local Administration § 5-10.1-4-6

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:

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.