Theta calculation

[counterparty]

This isn't a request for any action, just a question - as I prepare to replace the functions I've been using with those from FinAnSu, I've been comparing old to new, and all are spot-on (+/- precision) aside from theta.

Is there something I might be missing, or is this a known issue? (of course, I can provide examples, but doing so, unbidden, would be too much like me assigning you a task, which I will not do).