The first thing you need to do, is convert the string to a valid DateTime instance.
If you know your dates will always be in this format, you can do the following...
Dim mydate = DateTime.ParseExact("28 June 2018 (22:05)", "dd MMMM yyyy (HH:mm)", CultureInfo.InvariantCulture)
https://msdn.microsoft.com/en-us/library/w2sa9yss(v=vs.110).aspx
Once you've parsed the string into a valid DateTime instance, you can use all the normal date functions to do the comparisons.
I would first get the difference in minutes, like so...
Dim diffminutes = DateDiff(DateInterval.Minute, mydate, Now)
Then create a timespan like this...
Dim mytimespan = TimeSpan.FromMinutes(diffminutes)
Finally display the difference in hours and minutes like this...
Response.Write(mytimespan.ToString("hh:mm"))
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