You first have to get a proper DateTime value out of the string from the file:
using System.Globalization;
string dateString = ;
DateTime dateValue = DateTime.ParseExact(dateString, "dd/MM/yyyy HH:mm:ss", CultureInfo.CurrentCulture);
Then you can compare it to the current datetime. However, this gets a litlle tricky since you may want to compare only on a part of the values (for example, do not take into account the milliseconds). The implementation depends on your specification.