For two years, the Chicago Teachers’ Pension Fund used incorrect dates to calculate retirement benefits, records obtained by the Better Government Association and the Chicago Sun-Times show.

As a result of that error, a total of $2.78 million in overpayments went out to more than 200 retirees, the records show.

Now, the fund — which oversees pensions for more than 28,000 retired Chicago Public Schools teachers and administrators — is asking for its money back.

“They made a mistake — a big mistake,” says 68-year-old Jean Bosak, one of the 234 people who were paid too much.

Bosak says she’s struggling to pay back nearly $89,000 in overpayments. “I’m handicapped. I’m old. I live alone. Of course, it’s hard on me.”

The change resulting in the overpayments, made in 2012, was “an effort to simplify and automate pension processing,” according to pension fund records.

Jay Rehak, president of the 12-member board that oversees the pension fund, says it was “an error in interpretation and application of the Illinois Pension Code” regarding when pensioners began receiving their retiree benefits.

As a result, the pension fund’s administrators miscalculated benefits for the 234 retirees and mistakenly gave them lump-sum payments ranging from $566 to $217,185 as back pay between 2012 and 2014, when the error was discovered, records show.

After a pension fund staffer caught the mistake, retirees were contacted last year and told they had to return the money either by paying the amount back in full or in monthly increments through reduced pension payouts until the money was repaid.

Former third-grade teacher Sandra Inniss says she felt “sick” when she found out she had to pay back about $72,000.

“I just could not understand,” says Innis, who wondered about the lump-sum payment she got and says she tried to get someone at the pension fund to explain it. “I had been calling, I had been asking for a whole year, ‘What is this money for?’ ”

Inniss says she was repeatedly told the amount was correct and that it was a benefit she had earned, so she ultimately spent the money. Now, she is getting reduced pension each month until she has paid it all back.

“Before they started taking the money out of my check, I was still struggling,” Inniss says. “When you’re trying to make ends meet, this is very difficult.”

Clarice Berry, former president of the Chicago Principals and Administrators Association, says she was hit with a bill for about $15,000 owed to the fund.

“I was angry because I had done everything correctly,” Berry says.

Still, she considers herself lucky because she’s not among those who $100,000 or more.

“I couldn’t imagine someone calling me to say I owed $100,000 because of their error,” Berry says.

Chuck Burbridge, the pension fund’s executive director, says he recognizes the impact on retirees but says the collection efforts are necessary.

“There is no pleasure taking and doing this,” says Burbridge, who wasn’t working for the pension fund at the time the overpayments were made. “This is a tough, tough error to admit, recognize and correct. We are taking every step necessary to make sure we administer the fund according to statute appropriately because it’s a bad situation to be in.”

Rehak says pension trustees “acted prudently” in taking steps to recover the money.

So far, the fund has recovered $762,558.58, records show.

“As fiduciaries of the fund, the board cannot ignore its larger responsibilities, and we will work to recover all monies as quickly as possible,” Rehak says. “We also understand we have a fiduciary responsibility not only to our contributors and our annuitants but also to the taxpayers of Chicago and Illinois, who have the right to expect financial accountability.”

The teachers pension fund is funded mostly by taxpayers but also through employee contributions and investment income.

With $10.7 billion in assets, it’s 52 percent funded, according to its 2015 annual report. A minimum funding of 80 percent is generally considered healthy.

Mayor Rahm Emanuel recently committed to a $250 million property-tax hike to shore up money specifically for the teachers’ pension fund.

Katie Drews is an investigator for the Better Government Association.