Beginner: Data type conversion question

[flagg]

I am still fairly new to python and programming in general.  My
question is regarding data conversion, I am working on a script that
will edit dns zone files, one of the functions i wrote handles
updating the serial number.
Our zone files use the date as the first part of the serial and a two
digit integer as the last two.

i.e. 2009011501.  The next update would be 2009011502, etc
Here is the function I wrote, I am using dnspython for reading in zone
files as Zone "objects".  Because dnspython's built-in serial updater
will not work with how we format our serial's, I have to re-write it.

def checkSerial():
    """
    Checks the current 'date' portion of the serial number and
    checks the current 'counter'(the two digit number at the end of
    the serial number), then returns a complete new serial
    """
    currentDate =  time.strftime("%Y""%m""%d", time.localtime())
    for (name, ttl, rdata) in zone.iterate_rdatas(SOA):
        date = str(rdata.serial)[0:8]
        inc = str(rdata.serial)[8:10]
    if date == currentDate:
        int(inc) + 1
        print inc
        newInc = str(inc).zfill(2)
        serial = date + newInc
        print "date is the same"
        return serial
    elif date < currentDate:
        newInc = "01".zfill(2)
        serial = currentDate + newInc
        print "date is different"
        return serial

Through all of this I am doing a lot of data type conversion. string -
> integer, integer back to string, etc.  Is this an efficient way of
handling this?  I have to perform basic addition on the "inc"
variable, but also need to expose that value as a string.   What I
have above does work, but I can't help but think there is a more
efficient way. I guess I am not used to data types being converted so
easily.