08-26-2009;08-27-2009;08-29-2009

for element in iter:
    # Look for a tag called "Timeinfo"
    if element.tag == "timeinfo":
        tree = root.find(".//timeinfo")
        # Clear all tags below "timeinfo"
        tree.clear()
        element.append(ET.Element("mdattim"))
        child1 = ET.SubElement(tree, "sngdate")
        child2 = ET.SubElement(child1, "caldate1")
        child3 = ET.SubElement(child1, "caldate2")
        child4 = ET.SubElement(child1, "caldate3")
        child2.text = FIRST DATE VARIABLE GOES HERE
        child2.text = SECOND DATE VARIABLE GOES HERE
        child2.text = THIRD DATE VARIABLE GOES HERE

>>> a="08-26-2009;08-27-2009;08-29-2009"
>>> a_split = a.split(';')
>>> a_split
['08-26-2009', '08-27-2009', '08-29-2009']

child2.text, child3.text, child4.text = three_dates_text.split(';')

children =  [ET.SubElement(tree, "sngdate")]
children += [ET.SubElement(children[0], "caldate%s" % i) for i in xrange(3)]

dates = "08-26-2009;08-27-2009;08-29-2009"

for i, d in enumerate(dates.split(";")):
    children[i+1].date = d