What's a good way to check if two datetimes are on the same calendar day in TSQL?

Question

Here is the issue I am having: I have a large query that needs to compare datetimes in the where clause to see if two dates are on the same day. My current solution, which suck

Answer 1

You pretty much have to keep the left side of your where clause clean. So, normally, you'd do something like:

WHERE MyDateTime >= @activityDateMidnight 
      AND MyDateTime < (@activityDateMidnight + 1)

(Some folks prefer DATEADD(d, 1, @activityDateMidnight) instead - but it's the same thing).

The TimeZone table complicates matter a bit though. It's a little unclear from your snippet, but it looks like t.TheDateInTable is in GMT with a Time Zone identifier, and that you're then adding the offset to compare against @activityDateMidnight - which is in local time. I'm not sure what ds.LocalTimeZone is, though.

If that's the case, then you need to get @activityDateMidnight into GMT instead.