It's difficult to tell without seeing the workbook itself - but it could be because the DATEPART calculations will be returning 1 at the beginning of 2018, and 52 at the end of 2017. So "(DATEPART('week', [Ticket Completed]) - DATEPART('week', [Ticket Received]))" for a project started at the end of last year and completed at the beginning of this year would return -51.
If that's not it, could you post a .twbx?
I attached the workbook to the post.
see the link to year end effects on the week number in Tableau
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It looks like it is stemming from the problem I mentioned above. The DATEPART elements of the calculation don't work across two years, because the 'week' datepart resets to one.
It looks like you're not being affected by the week 53 issue that Jim linked to - as none of your tickets were opened in the last week of the year.
Unfortunately, these weekday calculations can get quite complicated in Tableau. Have a look at this thread which offers some possible solutionsRe: DateDiff for Working days only (the approach you're using is also proposed, but somebody points out that it might break when calculating across two calendar years).