How do I reassign a variable in XSLT?

I want to be able to do some basic reassignments to variables in XSLT. How can this be achieved?

I just want to be able to convert this to XSLT (ignoring the appendMonthWithZero() function):

if(currentMonth + count > 12) //If we get past December onto a new year we need to reset the current month back to 01
{
    currentMonth = (currentMonth + count) - 12; //e.g. 9 + 4 = 13, 13 - 12 = 1 (January). Or 9 + 11 = 20, 20 - 12 = 8 (August)
    if(currentMonth < 10)
    {
        currentMonth = appendMonthWithZero();
    }
}

So far I have this in XSLT but it doesn't work. I am looping through this 12 times so I want to keep modifying currentMonth amongst other variables:

<xsl:if test="$currentMonth + $count &gt; 12">
    <xsl:param name="currentMonth" select="($currentMonth + $count) - 12"/>
</xsl:if>

This is essentially what I'm trying to do overall in pseudocode (http://pastebin.com/WsaZaKnC):

currentMonth = getCurrentMonth();
actualDateWithZero = appendMonthWithZero();
docs = getFlightResults();
monthsArray = ['Jan', 'Feb', 'Mar'.......];

for(count = 0; count < 12; count++)
{   
    outboundMonth = subString(doc[count+1].getOutboundMonth());

    if(currentMonth + count > 12) //If we get past December onto a new year we need to reset the current month back to 01
    {
        currentMonth = (currentMonth + count) - 12; //e.g. 9 + 4 = 13, 13 - 12 = 1 (January). Or 9 + 11 = 20, 20 - 12 = 8 (August)
        if(currentMonth < 10)
        {
            currentMonth = appendMonthWithZero();
        }
    }

    //A price is available. 
    //Second check is for when we get past a new year
    if(currentMonth + count == outboundBoundMonth || currentMonth == outboundMonth)
    {
        //Get rest of data from doc etc etc
        //Set up divs etc etc
        //Get string month with displayed Month [Jan, Feb, Mar....]
    }

    //Else no price available for this month
    else
    {       
        //display price not available
        //Get string month with displayed Month [Jan, Feb, Mar....]
    }
}

XSLT is a declarative language, it doesn't use stateful variables. You need to work out how to express your output as a function of the input, rather than thinking of a way to give low-level procedural instructions to the computer.

In your case it seems very straightforward; just use different variables:

if(currentMonth + count > 12) {
    m2 = (currentMonth + count) - 12; 
    if (m2 < 10) then appendMonthWithZero(m2) else m2;
} else {
    currentMonth
}