Most of us have mastered basic counting skills in first and second grade — if not earlier.

Not so with the folks over at Service Employees International Union (SEIU) 721, whose incompetence dipped to a new low recently after the union was unable to properly determine an opt-out window period for Adrianna, a public worker in California.

After sending a letter to the union requesting she be released from membership and dues, Adrianna received a response notifying her that she would be forced to continue paying dues because her request hadn’t been received during an annual, union-set opt-out window.

This is a familiar ploy the Freedom Foundation is challenging in multiple lawsuits as unconstitutional (including a pending appeal to the U.S. Supreme Court).

But in this case, the opt-out window wasn’t merely unconstitutional. It was also miscalculated.

SEIU 721’s job here wasn’t difficult. While window periods vary from union to union, most are about 15 days in length and begin somewhere around 45 days before the date a public worker signed his or her most recent membership card.

If the union beancounters really didn’t want to take 10 seconds to calculate that, all they would have had to do is find a day convincingly close to the anniversary date of the card.

In Adrianna’s case, she had signed a membership card on three previous occasions — and the date the union provided her wasn’t even close to any of them. In fact, it was more than a year in length — which is impossible for an annually renewing window period.

Apparently, this eluded the scrutiny of anyone at SEIU 721’s office. It did catch the attention of the legal staff at the Freedom Foundation, however, after Adrianna contacted us asking for help.

The Freedom Foundation’s attorneys wasted no time sending out a letter to SEIU 721 demanding it explain the error. The union chalked it up to a “system error” and hastily refunded — with interest — the hundreds of dollars it had illegally taken.

The Freedom Foundation’s legal team exists for situations just like this — helping public workers get out of their unions when they are being unreasonable, dishonest or just plain incompetent.

Without this support to our outreach campaigns, many workers would have no recourse against unions that blatantly violate the law — or who otherwise can’t or won’t get their act together.