I guess you've to change the data type at data source (Source A) level. Once done then connect to tableau.
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I had a similar issue. This is how I solved it today:
Right click one of the data sources.
Click Edit Data Source...
Pull in the second data source to initiate the join.
Click the Inner Join icon.
If Tableau correctly recognizes your left data field, choose it.
On the right side, click the dropdown box.
At the bottom, there should be an option titled Create Join Calculation...
In my case, I typed DATETIME([Data_Field_Name]) because I was having issues with dates. You'll want to use the format you need.
The red exclamation error icon should go away.
Let me know if you experience similar results.