[dmoy]

It is interest free if the IRS pays you within N days of you filing. If they're slower, then they pay interest.

Where N is some value between like .... 30 and 90? I forget.

[loeg]

There are a number of different overpayment interest regimes[1]. Mine was paid based on time elapsed from the time of overpayment (overpaid quarterly estimated taxes).

[1]: https://www.irs.gov/payments/interest#pay