Lifecycle expiration header tests fail - Midnight UTC leads to incorrect day difference #652

The expiration date in the x-amz-expiration header is set to midnight (00:00) UTC of the expiration day, while start_time can be any time during the day. When you subtract these, the difference in days may be one less than expected because the expiration time is earlier (midnight) than start_time. This causes the days_to_expire check to fail even though the expiration is correct.

Below tests are failing:

FAILED s3tests_boto3/functional/test_s3.py::test_lifecycle_expiration_header_put - assert False
FAILED s3tests_boto3/functional/test_s3.py::test_lifecycle_expiration_header_head - assert False
FAILED s3tests_boto3/functional/test_s3.py::test_lifecycle_expiration_header_tags_head - assert False`

One fix is to adjust the start_time to midnight (00:00) UTC as well before doing the comparison. This aligns both times to midnight and makes the day difference calculation accurate, and the tests would pass.