With DATETIME variables, a common requirement is to add a number of minutes to a DATETIME.
Developers occasionally get stuck and the reason I believe is that Implicit Type conversions occasionally get you into bad habits. Sometimes you need to take a step back, use 3 lines of code instead of 1, then you can see the issue, and then see what the 1 line of code solution is.
With DATETIME addition, that can mean making sure that what you are attempting to add to the DATETIME is an INTERVAL, not a STRING. It should be DATETIME + INTERVAL = DATETIME
Read more here
https://4js.com/ask-reuben/ig-120/