HCL function format_date()

[johakoch]

Provide a format_date() function from stdlib (https://pkg.go.dev/github.com/zclconf/go-cty/cty/function/stdlib#FormatDate)

Example (by @afflerbach ):

    add_response_headers = {
      # format_date/date_format
      Last-Modified = format_date("Mon, 2 Jan 2006 15:04:05 MST") # Go
      Expires = format_date("EEE, d MMM yyyy HH:mm:ss z", unixtime() + 3600) # Java
    }

However, the FormatDate from stdlib (as used in https://developer.hashicorp.com/terraform/language/functions/formatdate) is

func FormatDate(format cty.Value, timestamp cty.Value) (cty.Value, error)

FormatDate reformats a timestamp given in RFC3339 syntax into another time syntax defined by a given format string.

[malud]

For the user we want to simplify the usage with specific parts of a date/timestamp.
Which standard we would prefer? e.g. YYYY-MM vs yyyy-mm ?

Does a go library exist which fulfills this requirements?

Which input formats are valid?
RFC3339 # 2006-01-02T15:04:05Z07:00
Unix
ISO8601
RFC850 = "Monday, 02-Jan-06 15:04:05 MST"
RFC1123 = "Mon, 02 Jan 2006 15:04:05 MST"

should we accept number and string?