You can set the variable to the current epoch time like this:
I used a number variable here, since the current time in milliseconds wound be a long
(or whatever similar data type Hubitat/Groovy ends up using for this). I do not believe these options are available for strings.
However, if you share what you're trying to accomplish (with the automation as a whole, not this specific step), someone might have other suggestions for how to get there. Often when people ask for this, there is a way to use a "Delay" or "Wait" to get the same effect instead, which may be easier.