Get the average from xml list

I have a list (USAGE_LIST) of data that contains some fields year and month and usage, i would like to get the average of the last 3 months of the usage field, I tried something like Calendar.getInstance() with SimpleDateFormat and getTime() but becuase i dont have really date, it doest work, what is the best way to do that, please an example. thanks and thanks for StackoverFlow

<USAGE_LIST>
            <TOTAL>
            <YEAR>2013</YEAR>
            <MONTH>10</MONTH>
            <USAGE>200</USAGE>
            </TOTAL>
            <TOTAL>
            <YEAR>2013</YEAR>
            <MONTH>11</MONTH>
            <USAGE>250</USAGE>
            </TOTAL>
            <TOTAL>
            <YEAR>2013</YEAR>
            <MONTH>12</MONTH>
            <USAGE>500</USAGE>
            </TOTAL>
            <TOTAL>
            <YEAR>2014</YEAR>
            <MONTH>1</MONTH>
            <USAGE>300</USAGE>
            </TOTAL>
</USAGE_LIST>