I find myself always falling into this trap.
We in Australia like most of the civilised world like our dates in DD/MM/YYYY format.
By default SSRS is MM/DD/YYYY which really only bothers me because it doesn’t recognise my data as valid dates! (13/12/2018 for example).
To fix this open your report, click on any artifact on the report (image, text box, table etc). Then in your Properties find “Report”.
In Report you’ll find “Language” which defaults to blank.