Date modification by filtered timezone

So when you say you want to modify dates to that timezone of the country do you mean that you want to have the Date/Time shift up or down based on where they are on the GMT?

My solution would be to firstly work out what Timezone your dates are recorded as.  For this example i'll presume that they represent a date/time in Greenwich (so GMT = 0).

Now create a Timezone table with 2 column, the Name of the timezone for your dropdown and the GMT modifier.  Here in Brisbane Australia, our modifier is GMT + 10.  Time data is stored as a decimal number between 0 and 1, so for Brisbane i'd need to store (10 / 24) as my modifier.  If you have a place that was GMT - 10, you'd need to store (-10 / 24).  This column we'll call [Timezone] and set as a number.

Now whenever you wish to display a date/time, your measure just needs to show [DateField] + MIN([Timezone]). Since your user will have selected a timezone, this will always return that selected value.  (If non is selected, you'll get the Lowest GMT value).