在python中使用lxml创建元素时出现“无效的标签名称”错误

我正在使用lxml制作xml文件，而我的示例程序是：

from lxml import etree
import datetime
dt=datetime.datetime(2013,11,30,4,5,6)
dt=dt.strftime('%Y-%m-%d')
page=etree.Element('html')
doc=etree.ElementTree(page)
dateElm=etree.SubElement(page,dt)
outfile=open('somefile.xml','w')
doc.write(outfile)

而且我得到以下错误输出：

dateElm=etree.SubElement(page,dt)
  File "lxml.etree.pyx", line 2899, in lxml.etree.SubElement (src/lxml/lxml.etree.c:62284)
  File "apihelpers.pxi", line 171, in lxml.etree._makeSubElement (src/lxml/lxml.etree.c:14296)
  File "apihelpers.pxi", line 1523, in lxml.etree._tagValidOrRaise (src/lxml/lxml.etree.c:26852)
ValueError: Invalid tag name u'2013-11-30'

I thought it of a Unicode Error, so tried changing encoding of 'dt' with codes like

str(dt)

unicode(dt).encode('unicode_escape')

dt.encocde('ascii','ignore')

dt.encode('ascii','decode')

and some others also, but none worked and same error msg generated.