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

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.