What you did was defining a page-sepcific parameter. For display purposes you need to use .Params.readingtime in your template (assuming that your post is the current content otherwise you need to assign the post to a variable depending on your needs). The official .ReadingTime variable is provided by hugo for each post and it’s value it automatically calculated so you cannot overwrite it.
By the way, you can also calculate the reading time yourself if you want. That way it’s possible to use a custom words per minutes to estimate reading time. Or use seconds in the output (like the readingtime front matter variable from the first post seems to do).
This Hugo template code outputs the reading time in minutes and seconds, assuming 220 words per minute:
Thanks, that’s of course correct. I unfortunately can’t update the post above anymore, but with .CountWords the first line of the above example becomes: