Basically there ar three methods to do:
Create a calculated field in your data that shows the value of the Now() function. You have to float that in its own worksheet like you did. But, you have to consider turning on the auto-refresh for the workbook so that the date/time is updated occasionally. It will show Now() by default from whenever the Tableau data source was last refreshed.
Look in Tableau's Postgres database for the last refresh time and put that on a worksheet and float it in a dashboard, if you want to use data extract.
It just depends on the specific needs. It will have to be worksheet based though; a dashboard title box cannot show anything bound to data. Use the floating worksheet to make it look like it's part of the title.