How to use timestamp() and merge() functions with tags

If the timestamp() function is added to the tags, then it destroys all of the default tags.

In order to prevent this, use the timestamp() function under a locals block and reference the locals block in the tags.

Example: Below example depicts the use of timestamp() , tags and along with merge().

provider "aws" {
  region = "us-west-2"
}
variable "extra_tags" {
  default = {
    tags = {
      Enviroment = "tags-test-env"
    }
  }
}
data "aws_caller_identity" "current" {}
locals {
  computed_tags = {
    LastModifiedTime = "${timestamp()}"
    LastModifiedBy   = "${data.aws_caller_identity.current.arn}"
  }
  merged_tags = merge(local.computed_tags, var.extra_tags)
}
output "map" {
  value = local.merged_tags
}
output "ignore_list" {
  value = ["${keys(local.merged_tags)}"]
}

Output:

+ ignore_list = [
      + [
          + "LastModifiedBy",
          + "LastModifiedTime",
          + "tags",
        ],
    ]
  + map         = {
      + LastModifiedBy   = "arn:aws:sts::323533494701:assumed-role/support_terraform_dev-developer/mtadiparthi@hashicorp.com"
      + LastModifiedTime = (known after apply)
      + tags             = {
          + Enviroment = "tags-test-env"
        }
    }

Documentation:

• Local Values
• timestamp Function
• merge Function