Please clarify what you mean by "the Wrong Date Type error". Do you mean the #VALUE error? Or do you mean a green triangle in the upper-corner of the cell?
If you are referring to a green triangle, that is just a warning, not an error. And yes, it is annoying.
I avoid them by disabling "background error checking", a misnomer. Click File > Options> Formulas, and uncheckmark "Enable background error checking". Also click Reset Ignored Errors to clear existing warnings.
If you see a #VALUE error....
Yes, you can write =G3-E3 instead of =SUM(G3-E3). And you should.
But I would be surprised if that remedies the problem.
If G3 and/or E3 is text that Excel can interpret as a number, G3-E3 will work just fine in both formulas.
The #VALUE error means that G3 and/or E3 is text that Excel __cannot__ interpret as a number.
Usually, Excel tolerates leading and trailing regular spaces (ASCII 32). So that should __not__ cause a #VALUE error in this context.
But with copy-and-paste, the problem is often with non-breaking spaces (ASCII 160). Much less often, there are other extraneious characters, notably tab characters (ASCII 9).
For quick solution, try entering formulas of the form =CLEAN(TRIM(SUBSTITUTE(G3,CHAR(160),""))) into a cell, then copy the cell and paste-value into G3. Do the same with E3.
I expect that will remedy the immediate problem.
But the long-term solution is to avoid the problem altogether. To that end, we would might need more information. LMK.
If that does not remedy the problem, it is best for us to see the Excel file.
Upload an example Excel file (redacted) that demonstrates the problem to a file-sharing website. I like box.net/files; others like dropbox.com. You might like onedrive.live.com because it shares the same login as this forum.
In any case, post the public download URL in a response. First, test the download URL, being careful to close all windows that might share the same login.