How to append timestamp to s3 bucket name in suffix using cloudformation template?

One way to do it is to write your own CloudFormation Macro or use an existing one. Have a look at the example Macros at https://github.com/aws-cloudformation/aws-cloudformation-macros to see what's possible. The PyPlate macro there may suit your needs without you having to write a new Macro.

Another option is the write a Custom Resource. If you do, I strongly recommend making use of something like crhelper (https://github.com/aws-cloudformation/custom-resource-helper) - it makes it so much easier.

I would not recommend creating a custom resource for this at all. Indeed a macro is more appropriate for what you are trying to do. But I would still recommend to stick to what CloudFormation "native" has to offer in this particular use-case for the bucket name: have a parameter for the bucket name. Personally though, I much prefer to to add region and account number with the native functions. The only parameter you need is a prefix (or suffix) to the bucket name. Hope this helps :)