I figured precedence should cast @txt to some form of datetime. But I didn't like any of the answers, because I didn't think you could add two datetimes together. The reference (https://technet.microsoft.com/en-us/library/ms178565(v=sql.105).aspx) says any of the numeric types except bit, and that it cannot be used with date, time, datetime2, or datetimeoffset. It doesn't specifically mention datetime.
You can add a number to a datetime, which adds that number of days, but adding two datetimes doesn't make sense. I'm going to postulate that the expression evaluator is converting one of the datetimes to the integer which represents its number-of-days-since-the beginning-of-time, and then adding that integer to the other datetime. Which seems about right if you actually run the code and try to figure out the result you get.