Hey Paul,
This is an interesting question for a bunch of reasons but before I take you too far away from the technical question you’re asking let me start by giving you an answer that you are most likely expecting.
Writing this function in Tulip’s expression editor is possible but very annoying and unreadable. The expression editor uses some version of moment.js I think, and even that community uses plugins for this calculation. In Tulip’s case (depending on when and where you want to calculate it) I’d use @Pete_Hartnett 's custom widget method Get working hours between two dates and times. Nowadays you could also use automations and split out the expression logic so it’s easier to follow.
You might come back saying “well what about days where we only worked 6 hours or days where we did overtime?”. And this is where I’ll take you away from your original question and get at the why of it all…
What are you doing this for? Sometimes we think of lead time from the consuming location perspective, for the purpose of materials planning. How long between when I order to when I receive. I’m assuming you aren’t looking at lead time for this purpose, but instead to understand how long it takes you to process things from start to finish in a cell you own. This is a good lagging indicator of performance but I wonder how well you could dig into it for problem solving. For example, let’s say your lead time balloons out of control because a machine went down and now WIP is piling up. Lead time as a measurement goes up no matter how you calculate it and no amount of cutting days out of it will help you reason that its because a machine went down and WIP has gone up. Does this example make sense?
Maybe another way to say it is, if you are looking at lead time from the perspective of a customer I would suggest keeping the fat in the measurement and if you are doing it from the perspective of the supplier then I’d be interested in lead times from process to process and other metrics like WIP, availability, etc.
Anyway, I just like thinking and talking about this stuff. Hope that link was helpful! And if you do want to find a way to calculate lead time this way including all sorts of variables I’d be willing to help you think it through, I like technical challenges for technical challenges sake, too.