There are two ways to do this.
The first one is interactive, using
CTRL-R = in the Ex command-line to enter an expression and evaluate it.
Start by typing the command as usual:
:w myFile-, then press
=. You'll now have a
= prompt where you can enter an expression. Enter
strftime("%F") and press "Enter", Vim will evaluate that to 2020-03-27 (assuming that's today's date) and will now display the command-line as
:w myFile-2020-03-27, which you can then execute or further edit as you like.
Another option is to use the
:execute command to execute a string as an Ex command. In which case you can simply use:
:exe 'w myFile-'.strftime("%F")
. is the string concatenation operator here. This latter form with
:execute is particularly useful in Vim scripts or functions.