diff --git a/warcprox/warc.py b/warcprox/warc.py
index 21d0f5d..94fe137 100644
--- a/warcprox/warc.py
+++ b/warcprox/warc.py
@@ -34,6 +34,9 @@ class WarcRecordBuilder:
         self.digest_algorithm = digest_algorithm
         self.base32 = base32
 
+    def format_warc_date(self, dt):
+        return dt.strftime('%Y-%m-%dT%H:%M:%SZ').encode('ascii')
+
     def _build_response_principal_record(self, recorded_url, warc_date):
         """Builds response or revisit record, whichever is appropriate."""
         if hasattr(recorded_url, "dedup_info") and recorded_url.dedup_info:
@@ -70,7 +73,7 @@ class WarcRecordBuilder:
 
     def build_warc_records(self, recorded_url):
         """Returns a tuple of hanzo.warctools.warc.WarcRecord (principal_record, ...)"""
-        warc_date = warctools.warc.warc_datetime_str(recorded_url.timestamp)
+        warc_date = self.format_warc_date(recorded_url.timestamp)
 
         if recorded_url.response_recorder:
             principal_record = self._build_response_principal_record(recorded_url, warc_date)
@@ -98,7 +101,7 @@ class WarcRecordBuilder:
         content_length=None):
 
         if warc_date is None:
-            warc_date = warctools.warc.warc_datetime_str(datetime.datetime.utcnow())
+            warc_date = self.format_warc_date(datetime.datetime.utcnow())
 
         record_id = warctools.WarcRecord.random_warc_uuid()
 
@@ -175,7 +178,7 @@ class WarcRecordBuilder:
         return output
 
     def build_warcinfo_record(self, filename):
-        warc_record_date = warctools.warc.warc_datetime_str(datetime.datetime.utcnow())
+        warc_record_date = self.format_warc_date(datetime.datetime.utcnow())
         record_id = warctools.WarcRecord.random_warc_uuid()
 
         headers = []