I have a Home Depot credit card that is supported by TidewaterFinance. I was charged interest on my purchase even though I had zero balance 2 cycles ago and never received a bill last month because I had zero balance. When I called they said credit agreement says interest will begin 25 days post-purchase. Is that legal?
It all depends on what your credit card agreement says. Most credits cards in the US have a "grace period" of at least 21 days, with 25 days being the most common. You mentioned 25 days, but you didn't specify if your card has a "25 day grace period". If it does, then generally this means your June 1 statement would not accrue any interest until the due date on June 25 if you had paid your previous 2 statements in full by their due dates.
I have found that the Home Depot credit card with Citibank has a 25 day grace period, but I cannot find any info about the card with Tidewater Finance. If you can find your agreement I would search for the words 25 day "grace period" vs 25 days "after/post purchase". If the latter then sadly their math is probably correct. Whether it's legal or not is a question for another forum, though I'd be surprised if Home Depot would be affiliated with a company that is breaking the law.
As a side note, if there is a specified "grace period", this would be a pretty strange mistake for an automated system to make, so the most likely scenario is that you carried a small balance and didn't realize it, and doing this negates the grace period. If it turns out that this is what happened, and if the amount carried over is less than the interest charged, you could ask for a one-time waiver on the interest and maybe they'll be nice and grant it.