S3: Metadata Last Modified Time is Null

I'm retrieving a bunch of files from S3 and I am trying to take action on these files based on their last modified time. However, when trying to retrieve the Last Modified time from the ObjectMetadata, it says that the value is null. When looking at the API it says that the metadata such as "creation date" and "last modified time" cannot be modified by the users, so how is it possible for something like that to be null? The API says that "Metadata such as object creation date is system controlled where only Amazon S3 can modify the value."

Any help would be much appreciated! Thanks in advance.