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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_backup_file_time_tag(file_name, custom_prefix="backup"):

 """ Returns a datetime object computed from a file name string, with
 formatting based on DATETIME_FORMAT."""

    name_string = file_name[len(custom_prefix):]
    time_tag = name_string.split(".", 1)[0]
    return datetime.strptime(time_tag, DATETIME_FORMAT)