diff --git a/linkcheck/checker/httpurl.py b/linkcheck/checker/httpurl.py
index 62d30594..244fd441 100644
--- a/linkcheck/checker/httpurl.py
+++ b/linkcheck/checker/httpurl.py
@@ -492,7 +492,8 @@ class HttpUrl (internpaturl.InternPatternUrl, proxysupport.ProxySupport):
             else:
                 self.set_result(_("OK"))
         modified = rfc822.parsedate(self.getheader('Last-Modified', u''))
-        self.modified = datetime.utcfromtimestamp(time.mktime(modified))
+        if modified:
+            self.modified = datetime.utcfromtimestamp(time.mktime(modified))
 
     def _try_http_response (self):
         """Try to get a HTTP response object. For reused persistent
diff --git a/linkcheck/checker/urlbase.py b/linkcheck/checker/urlbase.py
index 37ccf04c..ce11b592 100644
--- a/linkcheck/checker/urlbase.py
+++ b/linkcheck/checker/urlbase.py
@@ -185,7 +185,7 @@ class UrlBase (object):
         # content size
         self.size = -1
         # last modification time of content in HTTP-date format as specified in RFC2616 chapter 3.3.1
-        self.modified = u""
+        self.modified = None
         # download time
         self.dltime = -1
         # download size
@@ -1198,8 +1198,8 @@ class UrlBase (object):
           MIME content type for URL content.
         - url_data.level: int
           Recursion level until reaching this URL from start URL
-        - url_data.last_modified: unicode
-          Last modification date of retrieved page (or empty).
+        - url_data.last_modified: datetime
+          Last modification date of retrieved page (or None).
         """
         return dict(valid=self.valid,
           extern=self.extern[0],
diff --git a/linkcheck/logger/__init__.py b/linkcheck/logger/__init__.py
index 6102253c..df43b90c 100644
--- a/linkcheck/logger/__init__.py
+++ b/linkcheck/logger/__init__.py
@@ -437,6 +437,17 @@ class Logger (object):
         self.stats.addrinfo_stats = addrinfo_stats
         self.stats.downloaded_bytes = download_stats
 
+    def format_modified(self, modified, sep=" "):
+        """Format modification date if it's not None.
+        @param modified: modification date
+        @ptype modified: datetime or None
+        @return: formatted date or empty string
+        @rtype: unicode
+        """
+        if modified is not None:
+            return modified.isoformat(sep)
+        return u""
+
 
 # the standard URL logger implementations
 from .text import TextLogger
diff --git a/linkcheck/logger/csvlog.py b/linkcheck/logger/csvlog.py
index 30387e65..b8cdbfec 100644
--- a/linkcheck/logger/csvlog.py
+++ b/linkcheck/logger/csvlog.py
@@ -116,7 +116,7 @@ class CSVLogger (Logger):
         if self.has_part("level"):
             row.append(url_data.level)
         if self.has_part("modified"):
-            row.append(url_data.modified)
+            row.append(self.format_modified(url_data.modified))
         self.writerow(map(strformat.unicode_safe, row))
         self.flush()
 
diff --git a/linkcheck/logger/customxml.py b/linkcheck/logger/customxml.py
index d6b7d43b..1be55735 100644
--- a/linkcheck/logger/customxml.py
+++ b/linkcheck/logger/customxml.py
@@ -72,7 +72,7 @@ class CustomXMLLogger (xmllog.XMLLogger):
                 self.xml_tag(u"info", info)
             self.xml_endtag(u"infos")
         if url_data.modified and self.has_part('modified'):
-            self.xml_tag(u"modified", url_data.modified)
+            self.xml_tag(u"modified", self.format_modified(url_data.modified))
         if url_data.warnings and self.has_part('warning'):
             self.xml_starttag(u"warnings")
             for tag, data in url_data.warnings:
diff --git a/linkcheck/logger/html.py b/linkcheck/logger/html.py
index d30b1bbd..45b280e7 100644
--- a/linkcheck/logger/html.py
+++ b/linkcheck/logger/html.py
@@ -221,7 +221,7 @@ class HtmlLogger (Logger):
 
     def write_modified(self, url_data):
         """Write url_data.modified."""
-        text = cgi.escape(url_data.modified.isoformat(" "))
+        text = cgi.escape(self.format_modified(url_data.modified))
         self.writeln(u'<tr><td valign="top">' + self.part("modified") +
             u"</td><td>"+text+u"</td></tr>")
 
diff --git a/linkcheck/logger/sitemapxml.py b/linkcheck/logger/sitemapxml.py
index 9981ae52..f7f73814 100644
--- a/linkcheck/logger/sitemapxml.py
+++ b/linkcheck/logger/sitemapxml.py
@@ -67,34 +67,33 @@ class SitemapXmlLogger (xmllog.XMLLogger):
         Update accounting data and determine if URL should be included in the sitemap.
         """
         self.stats.log_url(url_data, do_print)
-        # ignore the do_print flag and determine ourselves if we filter the url
-        if (url_data.valid and
-            url_data.url.startswith((u'http:', u'https:')) and
-            url_data.url.startswith(self.prefix) and
-            url_data.content_type in ('text/html', "application/xhtml+xml")):
-            self.log_url(url_data)
-
-    def log_url (self, url_data):
-        """
-        Log URL data in sitemap format.
-        """
+        # initialize prefix and priority
         if self.prefix is None:
-            # first URL (ie. the homepage) gets priority 1.0 per default
             self.prefix = url_data.url
+            # first URL (ie. the homepage) gets priority 1.0 per default
             priority = 1.0
         else:
             # all other pages get priority 0.5 per default
             priority = 0.5
         if self.priority is not None:
             priority = self.priority
+         # ignore the do_print flag and determine ourselves if we filter the url
+        if (url_data.valid and
+            url_data.url.startswith((u'http:', u'https:')) and
+            url_data.url.startswith(self.prefix) and
+            url_data.content_type in ('text/html', "application/xhtml+xml")):
+            self.log_url(url_data, priority=priority)
+
+    def log_url (self, url_data, priority=None):
+        """
+        Log URL data in sitemap format.
+        """
         self.xml_starttag(u'url')
         self.xml_tag(u'loc', url_data.url)
         if url_data.modified:
-            modified = get_sitemap_modified(url_data.modified)
-            if modified:
-                self.xml_tag(u'lastmod', modified)
+            self.xml_tag(u'lastmod', self.format_modified(url_data.modified, sep="T"))
         self.xml_tag(u'changefreq', self.frequency)
-        self.xml_tag(u'priority', "%.1f" % priority)
+        self.xml_tag(u'priority', "%.2f" % priority)
         self.xml_endtag(u'url')
         self.flush()
 
@@ -106,13 +105,3 @@ class SitemapXmlLogger (xmllog.XMLLogger):
         self.xml_end_output()
         self.close_fileoutput()
 
-
-def get_sitemap_modified(modified):
-    """Reformat UrlData modified string into sitemap format specified at
-     http://www.w3.org/TR/NOTE-datetime.
-    @param modified: last modified time
-    @ptype modified: datetime object with timezone information
-    @return: formatted date
-    @rtype: string
-    """
-    return modified.isoformat('T')
diff --git a/linkcheck/logger/sql.py b/linkcheck/logger/sql.py
index 6556c9bd..d352401f 100644
--- a/linkcheck/logger/sql.py
+++ b/linkcheck/logger/sql.py
@@ -120,7 +120,7 @@ class SQLLogger (Logger):
                'cached': intify(url_data.cached),
                'separator': self.separator,
                "level": url_data.level,
-               "modified": url_data.modified,
+               "modified": sqlify(self.format_modified(url_data.modified)),
               })
         self.flush()
 
diff --git a/linkcheck/logger/text.py b/linkcheck/logger/text.py
index 79e316f0..7b9051ac 100644
--- a/linkcheck/logger/text.py
+++ b/linkcheck/logger/text.py
@@ -181,7 +181,7 @@ class TextLogger (Logger):
     def write_modified(self, url_data):
         """Write url_data.modified."""
         self.write(self.part("modified") + self.spaces("modified"))
-        self.writeln(url_data.modified.isoformat(" "))
+        self.writeln(self.format_modified(url_data.modified))
 
     def write_warning (self, url_data):
         """Write url_data.warning."""