Gurbs is indicating how to get a singular field value as a maximum.
I believe you want all of the values from the row with the max date (for that group/site).
Your approach of using sorting and then placing the field on the GH works fine.
When you place fields in a header or footer they will display the first(header) or last (footer) record that was before the header or footer weas printed.
Remember that duppressing rows (or fields) do not exclude them from the report or any calculations that you are doing.
YOu can alos use Gurbs suggestion in conjunction with a suppression formula, assuming you only have opne row that meets your maximum.
The suppression formula would be something like
datetimefield<>maximum(datetimefield,groupfield)