I don't think $[unixtime ] works in MFM per the spec, as implemented in Misskey's playground, or on FireFish. @julia is an expert so she can correct me but I believe the parser requires a function name and at least some content. You can't just have a dangling space like that or a function that has no content so $[unixtime] shouldn't work either.
If it did work, it would be something like $[unixtime now] or $[unixtime.year=2024,month=1,day=24 UTC].
Maybe Sharkey does something different but I can't see any docs to indicate that.