Using s3fs with no 'LastModified' key info

Start with s3fs/core.py at the info() code reported at line 447, then follow how open() obtains object metadata. Reproduce the Ceph response without LastModified and compare it with the metadata expected by S3FileSystem. Done means the reported open() operation handles this response without the KeyError while preserving existing ls(), mkdir(), isfile(), and isdir() behavior.

I have a Ceph based Object store with S3, and I am trying to use s3fs.

The metadata of the objects do not seem to have 'LastModified' entry, which was identified using Boto3's s3.Client.head_object().
Here's a sample metadata response of one of the objects

{'ResponseMetadata': {'RequestId': 'tx0000000000000067d7a8a-005c3dce53-489594c6-in-chennai', 'HostId': '', 'HTTPStatusCode': 200, 'HTTPHeaders': {'server': 'openresty/1.13.6.2', 'date': 'Tue, 15 Jan 2019 12:13:07 GMT', 'content-type': 'application/octet-stream', 'accept-ranges': 'bytes', 'etag': '"ed076287532e86365e841e92bfc50d8c"', 'content-length': '12', 'x-amz-request-id': 'tx0000000000000067d7a8a-005c3dce53-489594c6-in-chennai', 'expires': 'Thu, 31 Dec 2037 23:55:55 GMT', 'cache-control': 'max-age=315360000', 'x-e-id': '10.34.34.118', 'x-elb-id': '10.34.34.118', 'x-elb-app-server': '10.34.105.188'}, 'RetryAttempts': 0}, 'AcceptRanges': 'bytes', 'ContentLength': 12, 'ETag': '"ed076287532e86365e841e92bfc50d8c"', 'CacheControl': 'max-age=315360000', 'ContentType': 'application/octet-stream', 'Expires': datetime.datetime(2037, 12, 31, 23, 55, 55, tzinfo=tzutc()), 'Metadata': {}}

When I try to use s3fs operations like open() [To open a file for reading and writing], it gives the following error

Traceback (most recent call last):
  File "/Users/aman.garg/Downloads/test/s3fs-touch.py", line 11, in <module>
    with fs.open('amantestnotebook/notebook/write_file.txt', 'rb') as f:
  File "/Users/aman.garg/anaconda2/envs/s3contents/lib/python3.7/site-packages/s3fs/core.py", line 315, in open
    s3_additional_kwargs=kw)
  File "/Users/aman.garg/anaconda2/envs/s3contents/lib/python3.7/site-packages/s3fs/core.py", line 1102, in __init__
    info = self.info()
  File "/Users/aman.garg/anaconda2/envs/s3contents/lib/python3.7/site-packages/s3fs/core.py", line 1120, in info
    refresh=refresh, **kwargs)
  File "/Users/aman.garg/anaconda2/envs/s3contents/lib/python3.7/site-packages/s3fs/core.py", line 447, in info
    'LastModified': out['LastModified'],
KeyError: 'LastModified'

Operations like ls(), mkdir(),isfile(), isdir() are working as expected.

Is there a way to use S3FileSystem without the 'LastModified' metadata? What exactly is this LastModified entry used for?