prompt: We are currently solving the following issue within our repository. Here is the issue text:
  --- BEGIN ISSUE ---
  Parsing SEED: 'Date is required.' Warning
  Hi,
  
  Each time I want to read a dataless with different periods of time, I have this annoying warning message:
  
  ```
  from obspy.io.xseed import Parser
  from obspy import UTCDateTime
  Parser('http://geoscope.ipgp.fr/metadata/G/dataless.G.CAN.seed')
  /Users/bonaime/git/obspy/obspy/io/xseed/fields.py:374: UserWarning: Date is required. warnings.warn('Date is required.', UserWarning)
  ```
  
  Is there a nice way to avoid this warning ? I try that but it is not working
  
  ``` code
  from obspy.io.xseed import Parser
  from obspy import UTCDateTime
  Parser('http://geoscope.ipgp.fr/metadata/G/dataless.G.CAN.seed').get_paz('G.CAN.00.BHZ', datetime=UTCDateTime())
  
  ```
  
  and the result is
  
  ```
  /Users/bonaime/git/obspy/obspy/io/xseed/fields.py:374: UserWarning: Date is required.
    warnings.warn('Date is required.', UserWarning)
  Out[1]:
  {u'digitizer_gain': 1677720.0,
   u'gain': 1.24658e+17,
   u'poles': [(-0.0120768+0.011706j),
    (-0.0120768-0.011706j),
    (-36.4684+66.8452j),
    (-36.4684-66.8452j),
    (-29.8656+380.54j),
    (-29.8656-380.54j),
    (-12145.6+0j),
    (-12145.6+0j)],
   u'seismometer_gain': 3450.0,
   u'sensitivity': 5788280000.0,
   u'zeros': [0j, 0j]}
  ```
  
  
  --- END ISSUE ---
  
  Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
  --- BEGIN FILES ---
  Path: `obspy/io/xseed/blockette/blockette051.py`
  Content:
  ```
  1 # -*- coding: utf-8 -*-
  2 from __future__ import (absolute_import, division, print_function,
  3                         unicode_literals)
  4 from future.builtins import *  # NOQA
  5 
  6 from .blockette import Blockette
  7 from ..fields import Integer, VariableString
  8 
  9 
  10 class Blockette051(Blockette):
  11     """
  12     Blockette 051: Station Comment Blockette.
  13 
  14     Sample:
  15     05100351992,001~1992,002~0740000000
  16     """
  17 
  18     id = 51
  19     name = "Station Comment"
  20     fields = [
  21         VariableString(3, "Beginning effective time", 1, 22, 'T'),
  22         VariableString(4, "End effective time", 1, 22, 'T', optional=True),
  23         Integer(5, "Comment code key", 4, xpath=31),
  24         Integer(6, "Comment level", 6, ignore=True)
  25     ]
  26
  ```
  --- END FILES ---
  
  Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.
  
  Here is an example:
  
  ```diff
  diff --git a/examples/server_async.py b/examples/server_async.py
  --- a/examples/server_async.py
  +++ b/examples/server_async.py
  @@ -313,4 +313,4 @@
   
   
   if __name__ == "__main__":
  -    asyncio.run(run_async_server("."), debug=True)
  +    asyncio.run(run_async_server(), debug=True)
  diff --git a/examples/server_sync.py b/examples/server_sync.py
  --- a/examples/server_sync.py
  +++ b/examples/server_sync.py
  @@ -313,5 +313,5 @@
   
   
   if __name__ == "__main__":
  -    server = run_sync_server(".")
  +    server = run_sync_server()
       server.shutdown()
  
  ```
  
golden_diff: diff --git a/obspy/io/xseed/blockette/blockette051.py b/obspy/io/xseed/blockette/blockette051.py
  --- a/obspy/io/xseed/blockette/blockette051.py
  +++ b/obspy/io/xseed/blockette/blockette051.py
  @@ -19,7 +19,7 @@
       name = "Station Comment"
       fields = [
           VariableString(3, "Beginning effective time", 1, 22, 'T'),
  -        VariableString(4, "End effective time", 1, 22, 'T', optional=True),
  +        VariableString(4, "End effective time", 0, 22, 'T', optional=True),
           Integer(5, "Comment code key", 4, xpath=31),
           Integer(6, "Comment level", 6, ignore=True)
       ]
  
verification_info: {"golden_diff": "diff --git a/obspy/io/xseed/blockette/blockette051.py b/obspy/io/xseed/blockette/blockette051.py\n--- a/obspy/io/xseed/blockette/blockette051.py\n+++ b/obspy/io/xseed/blockette/blockette051.py\n@@ -19,7 +19,7 @@\n     name = \"Station Comment\"\n     fields = [\n         VariableString(3, \"Beginning effective time\", 1, 22, 'T'),\n-        VariableString(4, \"End effective time\", 1, 22, 'T', optional=True),\n+        VariableString(4, \"End effective time\", 0, 22, 'T', optional=True),\n         Integer(5, \"Comment code key\", 4, xpath=31),\n         Integer(6, \"Comment level\", 6, ignore=True)\n     ]\n", "issue": "Parsing SEED: 'Date is required.' Warning\nHi,\n\nEach time I want to read a dataless with different periods of time, I have this annoying warning message:\n\n```\nfrom obspy.io.xseed import Parser\nfrom obspy import UTCDateTime\nParser('http://geoscope.ipgp.fr/metadata/G/dataless.G.CAN.seed')\n/Users/bonaime/git/obspy/obspy/io/xseed/fields.py:374: UserWarning: Date is required. warnings.warn('Date is required.', UserWarning)\n```\n\nIs there a nice way to avoid this warning ? I try that but it is not working\n\n``` code\nfrom obspy.io.xseed import Parser\nfrom obspy import UTCDateTime\nParser('http://geoscope.ipgp.fr/metadata/G/dataless.G.CAN.seed').get_paz('G.CAN.00.BHZ', datetime=UTCDateTime())\n\n```\n\nand the result is\n\n```\n/Users/bonaime/git/obspy/obspy/io/xseed/fields.py:374: UserWarning: Date is required.\n  warnings.warn('Date is required.', UserWarning)\nOut[1]:\n{u'digitizer_gain': 1677720.0,\n u'gain': 1.24658e+17,\n u'poles': [(-0.0120768+0.011706j),\n  (-0.0120768-0.011706j),\n  (-36.4684+66.8452j),\n  (-36.4684-66.8452j),\n  (-29.8656+380.54j),\n  (-29.8656-380.54j),\n  (-12145.6+0j),\n  (-12145.6+0j)],\n u'seismometer_gain': 3450.0,\n u'sensitivity': 5788280000.0,\n u'zeros': [0j, 0j]}\n```\n\n", "before_files": [{"content": "# -*- coding: utf-8 -*-\nfrom __future__ import (absolute_import, division, print_function,\n                        unicode_literals)\nfrom future.builtins import *  # NOQA\n\nfrom .blockette import Blockette\nfrom ..fields import Integer, VariableString\n\n\nclass Blockette051(Blockette):\n    \"\"\"\n    Blockette 051: Station Comment Blockette.\n\n    Sample:\n    05100351992,001~1992,002~0740000000\n    \"\"\"\n\n    id = 51\n    name = \"Station Comment\"\n    fields = [\n        VariableString(3, \"Beginning effective time\", 1, 22, 'T'),\n        VariableString(4, \"End effective time\", 1, 22, 'T', optional=True),\n        Integer(5, \"Comment code key\", 4, xpath=31),\n        Integer(6, \"Comment level\", 6, ignore=True)\n    ]\n", "path": "obspy/io/xseed/blockette/blockette051.py"}], "after_files": [{"content": "# -*- coding: utf-8 -*-\nfrom __future__ import (absolute_import, division, print_function,\n                        unicode_literals)\nfrom future.builtins import *  # NOQA\n\nfrom .blockette import Blockette\nfrom ..fields import Integer, VariableString\n\n\nclass Blockette051(Blockette):\n    \"\"\"\n    Blockette 051: Station Comment Blockette.\n\n    Sample:\n    05100351992,001~1992,002~0740000000\n    \"\"\"\n\n    id = 51\n    name = \"Station Comment\"\n    fields = [\n        VariableString(3, \"Beginning effective time\", 1, 22, 'T'),\n        VariableString(4, \"End effective time\", 0, 22, 'T', optional=True),\n        Integer(5, \"Comment code key\", 4, xpath=31),\n        Integer(6, \"Comment level\", 6, ignore=True)\n    ]\n", "path": "obspy/io/xseed/blockette/blockette051.py"}]}