Is there a tutorial on custom expressions, such as getting the current date in the format yyyy-mm-dd?
At present, there is no way to generate dates in a specific format using custom expressions.
However, you can write JavaScript code in the pre-script to obtain the current date in the desired format.
We are working on optimizing the logic of the data generator and will consider incorporating this feature in the near future.
This should be the expression you are looking for.
{{ $isoTimestamp | replace("T", " ") | substr(0, 19) }}
how you know $isoTimestamp variable? Is there any documentation? thank you 
oh it turns out like postman, thank you