The text field are designed to take in string inputs. It will return you whatever data is entered as string.
If you want to interpret this string, you have to write a function that parses the string in the manner you want it to be parsed. It is not recommended to use eval function with user inputs, as the user can then key in anything causing your program to break.
You have to decide what are the acceptable patterns of input and then write a parser that extracts these patterns for you.
In this case one of the input pattern you have is %i:%i. You can use regexp to extract these and other patterns.
stringvalue = '12:24';
out = regexp(stringvalue,'(?<d1>\d+):(?<d2>\d+)','names');
arr = str2double(out.d1):str2double(out.d2);