The post you are referring to is several years old.
The approaches you suggested work in later versions of the product. You can specify the grouping expression as = Fields.DateField.Date
which will group the data by the date only ignoring the time value.
Another approach to format the DateTime values is with ToString(formatString) or Format() built-in function.
Thus, test upgrading
to the latest version and check if this produces the expected result.
Step-by-step instructions on how to add a group are provided in How to: Add groups to Report
In case, the issue persists please send us the problematic report definition in a support ticket so we can check the settings and provide you further suggestions.
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