Browser Behaving differently to Desktop

Current Situation:
I have created a drop down in Excel Worksheet 'Charts' to display data for a specific time range from another sheet 'Comment Tracker'.
The Drop down looks at the following range which is created by using "Unique" to identify date entries and then "Sort" to create an ordered list of the unique entries. The Drop down list is created using a formula to ensure its size =OFFSET('Comment tracker'!$DF$3,0,0,COUNTA('Comment tracker'!$DF$3:$DF$101),1) Drop Down range currently contains Sep 22, Oct 22, Nov 22, Dec 22, Jan 23.
DropData.JPG

Problem Behavior
The Drop down works perfectly in Desktop but in Browser when some of the users click on the date cell, excel converts the content from for example Oct 22 to Oct 23. Even if user has cell formatted as custom number mmm yy. This happens even though that value doesn't exist in the drop down. The selection is then used in a formula to modify the data seen by a chart using Countifs to filter
DropSelect.JPG


Attempts to resolve I've searched settings to see if there is anything, checked all the formulas and everything is perfect. Formats are the same mmm yy everywhere.

I tried to originally post this on nanother forum but the forum policeman wouldn't allow me. I know not why :-( I'm hoping somebody here can point me inn the right direction please.