How to Get Difference between frommonth with fromyear compared to tomonth with toyear

This is a basic logic problem, which is easy to work out if you simply write it out.

From the example you have given, I assume you are working from month start to month end. I.e. Jan 2013 to Jan 2013 would be 1 month; Jan 2012 to Dec 2012 would be 1 year; Jan 2012 to Jan 2013 would be 1 year 1 month.

(Note that for this code it doesn't matter if your months are 1-based or 0-based).

int years = toYear - fromYear;
int months = toMonth - fromMonth + 1; // Add one, because we're assuming beginning of "fromMonth" to end of "toMonth" - i.e. if same month and year given for from and to, then this is 1 month.

// If toMonth is earlier than fromMonth then we'll have a negative number.
// Subtract a year and add 12 to the months.
// E.g. Dec 2012 to Jan 2013 = 1 month.  2013 - 2012 = 1; 1 - 12 (or 0 - 11) = -11 => 0 years, -11 + 12 = 1 month.
if (months < 0)
{
    --years;
    months += 12;
}
// Example tests:
// fromYear = 2006, fromMonth = Mar, toYear = 2007, toMonth June => 1 year 3 months.  (6 - 3 = 3; 2007 - 2006 = 1)
// fromYear = 2012, fromMonth = Mar, toYear = 2012, toMonth = Jun => 3 months. (6 - 3 = 3; 2012 - 2012 = 0)
// fromYear = 2012, fromMonth = Dec, toYear = 2013, toMonth = Jan => 1 month.  (1 - 12 + 1 = -11 => +12 = 1; 2013 -2012 - 1 = 0)
// fromYear = 2010, fromMonth = June, toYear = 2013, toMonth = Feb => 2 years 9 months. (2 - 6 + 1 = -3 => +12 = 3; 2013 - 2012 - 1 = 2)
// fromYear = 2010, fromMonth = Jan, toYear = 2013, toMonth = Jan => 3 years 1 month.  (1 - 1 + 1 = 1; 2013 - 2012 = 3)
// fromYear = 2010, fromMonth = Feb, toYear = 2013, toMonth = Jan => 3 years.  (1 - 2 + 1 = 0; 2013 - 2012 = 3)

After that you need to work out how you want to display this based on "years" and "months". A very simple example (note I've not used any globalization here, etc, and you would probably want to do things like check for the singular "year" and "month", etc.):

if (years > 0)
{
    if (months > 0)
    {
        display = string.Format("{0} years {1} months", years, months);
    }
    else
    {
        display = string.Format("{0} years", years);
    }
}
else
{
    display = string.Format("{1} months", months);
}

Regards,
Ian.