Cross Region Copy of Aurora Snapshots current_date = str(dt.now().strftime('%Y-%m-%d'))
target_cluster_snapshot_arn= current_date+'_Development_Aurora'

response = TARGET_CLIENT.copy_db_cluster_snapshot(
    SourceDBClusterSnapshotIdentifier=source_cluster_snapshot_arn,
    TargetDBClusterSnapshotIdentifier=target_cluster_snapshot_arn,
    KmsKeyId='arn:aws:kms:us-west-2:xxxxxxx:key/axxxxxx-e326-4df2-8274-73f87ff02f37',
    CopyTags=True,
    Tags=[
        {
            'Key': 'Deletion_Date',
            'Value': (dt.now() + datetime.timedelta(days=30)).strftime('%Y-%m-%d')
        },
    ],
    SourceRegion=SOURCE_REGION
)


While using the above code in Python I am getting the error as 

botocore.exceptions.ClientError: An error occurred (InvalidParameterValue) when calling the CopyDBClusterSnapshot operation: Invalid cluster snapshot identifier:  2018-05-22_Development_Aurora


But when i am hardcoding the value in TargerDBClusterSnapshotIdnetifier like below, its working fine

response = TARGET_CLIENT.copy_db_cluster_snapshot(
    SourceDBClusterSnapshotIdentifier=source_cluster_snapshot_arn,
    TargetDBClusterSnapshotIdentifier='PrashastTest',
    KmsKeyId='arn:aws:kms:us-west-2:xxxxxxx:key/xxxxxxb3-e326-4df2-8274-73f87ff02f37',
    CopyTags=True,
    Tags=[
        {
            'Key': 'Deletion_Date',
            'Value': (dt.now() + datetime.timedelta(days=30)).strftime('%Y-%m-%d')
        },
    ],
    SourceRegion=SOURCE_REGION
)


Any pointers why is this happening?
Your proposed snapshot-id is not syntactically valid.  When you tried hard-coding the value, you used a valid identifier.  

Underscores are not allowed, and the identifier can't begin with a number.

Here are the constraints on snapshot identifiers, from the API Reference:


  DBClusterSnapshotIdentifier
  
  The identifier of the DB cluster snapshot. This parameter is stored as a lowercase string.
  
  Constraints:
  
  
  Must contain from 1 to 63 letters, numbers, or hyphens.
  First character must be a letter.
  Cannot end with a hyphen or contain two consecutive hyphens.
  
  
  https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_CreateDBClusterSnapshot.html