Datediff works a little differently than you expect. It's only calculated based on the datepart you give it. So for example, if you did DATEDIFF('year',#2014-01-01#,#2016-12-31#), it would see 2016-2014 = 2, even though those dates are way more than 2 years apart. So in your case, you'd probably want to do your 'within two years' check with something like DATEDIFF('day',[Received UTC], [Delivered UTC]) <= 730.
Thanks, you're right. I'm looking to set an upper limit so DATEDIFF('year',#2014-01-01#,#2016-12-31#)=2 is not a problem, but DATEDIFF('year',#2015-12-31#,#2016-01-01#)=1 is a problem for my check. I switched to weeks.
That improves my code, but doesn't answer the question about the domain exceeding what should be the result of the calculation. Turns out I found a Tableau bug that was giving crazy results for some records. Support hasn't yet figured out what set of conditions create the crazy results.