Employer trying to clawback maternity leave

Location: California

I received approximately $35,000 in maternity leave benefits from my employer after giving birth. I signed a form stating that if I separate from employment within six months of returning to work, my maternity leave salary converts into a loan, repayable over five years with interest.

I notified HR of my intention to separate exactly six months after my return to work. However, they are now claiming that because I initially returned on a part-time basis, the six-month requirement is based on working 1,040 hours, not a calendar six months. That is not what the form says.

Am I legally required to repay them?