What is your data source? Maybe these dates are indeed of datetime type and you should only take care of formatting.
Well it is an excel extract from GoTomeeting. This data is from registration process for webinars. I can not change the source. Just the excel :-(
So... I do not know that app you exported data from but it would be worth checking if it is possible to change datetime string format (e.g. to ISO) that excel would recognize. The challenge here is also to retain GMT offset and I would recommend to use UTC standard if possible. CET would be GMT+01:00 while CEST GMT+02:00.
If you cannot do anything abut the above you have to convert those strings to datetime. Converting to local time is trivial - using dateparse or string manipulation and casting - but then they must be unified to match same GMT offset - preferably UTC, i.e. GMT+0. This can be done using a table with all the timezone used which I happen to have (last updated in the beginning of this year) and can share if you wish. But join condition would be timezone abbreviation and date range it applies to so you will have to do that in source (possible in excel) which is not supported by Tableau (up to 10.1). If this is one-off analysis and you just need to convert those times then you can share your data and I can do that for you.