Former employees of Seven-Eleven Japan Co.'s franchise stores on Wednesday gave vent to their anger and frustration on social media over the company's handling of unpaid overtime wages.

Japan's largest convenience store chain said Tuesday it did not pay a portion of overtime wages to at least 30,000 workers, totaling some 490 million yen ($4.5 million), after March 2012 due to a miscalculation in its payroll system. It said records only exist from that month onward.

Still, the company admitted that the problem dates back to its founding in the 1970s and promised to provide all the unpaid wages before March 2012, if the former employees can show their pay slips or other proofs of working records.

"I don't have such old pay slips," one of the former employees angrily wrote on a social media platform.

Another former employee said Seven-Eleven rejected his request as he only managed to find a bankbook showing salary payment records.

A number of people said in their online comments that Seven-Eleven had no intention of paying the old wages in the first place, although it has established a support window to deal with the issue.

Seven-Eleven has said the miscalculation was discovered in a probe at one of its franchisees by labor authorities in 2001, but it has neglected dealing with the issue or publicizing it.

The error affected 8,129 stores nationwide and resulted in unpaid wages totaling as much as 2.8 million yen for one employee, according to the company.

Some franchise store owners aired concerns that the company's way of dealing with the problem will likely make it even more difficult for them to secure workers at their shops, amid labor shortages in Japan.

To deal with the labor crunch, Seven-Eleven will close about 50 stores in Tokyo on New Year's Day on a trial basis, a source familiar with the plan said Wednesday.

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