Hi,
No, you're question is not stupid - in fact, the only stupid question is the one that is not asked
You're proposale could be correct, but in fact the issue is not coming from date in PHP but from XSLX reading library wich expect to deal with dates includng time.
For this library 2015-05-20 00:00:00 (as stored in XLSX file) corrected with location time can be (for instance with -4 from UTC) something like 2015-05-19 20:00:00, and was imported as 2015-05-19 in ProjeQtOr.
In my case (with +2 from UTC) same date was converted as 2015-05-20 02:00:00, and imported as 2015-05-20 (correct date).
Now, with new function to make PHP believe the locale time is UTC, the same date is alway retrieved as 2015-05-20 00:00:00 and so correctly imported.
For CSV import, there is not conversion depending on locale : date as 2015-05-20 00:00:00 is read and stored as is.
So, adding the command as you propose will have exacly NO effect
Hope that was clear.
Regards.