It seems to me that having a date-type column named `is_frobbed` is highly preferable to `frobbed_at` if your intent is for `IS NOT NULL` to mean frobbed.
Sure, it's a little weird to have boolean-sounding `is_frobbed` name for a date column, but the number of times that `frobbed_at IS [NOT] NULL` appears in the code is likely to dwarf the number of times you're inserting a date type into the `is_frobbed` column. I feel like there's always going to be someone (like me in this case) that's going to come across a `frobbed_at IS NOT NULL` case, notice that it's a date column and make the same flawed assumption I did. A similar thing will happen to the `is_frobbed` column too, but in that case trying to treat the date type as a boolean is going to make it obvious that you're not understanding the full context.
Frankly, if I was naming a column for this idiom and didn't have other constraints (like the Rails context), I would probably try to find a more exact and direct way to express it, if that's a little clunky. Maybe something like `frobbed_when_not_null`? But I don't love that. Honestly it may make more sense to have a actual boolean-valued `is_frobbed` column and an independent `frobbed_at` timestamp column that's populated by a trigger when the value of `is_frobbed` changes. I feel like "clunky but direct" beats out "elegant but misleading" in the long run, especially given the degree of "clunky" and "elegant" we're talking about here.
I don't expect to talk you out of it, and I'd fall in line with this in a Rails context too if that's the convention, but I think it's objectively poor design.
For what it's worth, if the `frobbed_at` convention usually intended to track the timestamp at which the frobbing happened, the good news is that my `AND frobbed_at <= NOW()` check would be unlikely to break anything in practice. Assuming that there's nothing weird going on with the timestamps, the frobbed_at dates will always be in the past anyway.