User activated in chat flow

If you are referring to the time that the subscriber was first subscribed to the bot, you can use the variable {{subscribed_at}}

If you want to get the time when the user activated any of the flow at any time, you can use the Function element.

On the function element, you can use this snippet:

return (new Date()).toLocaleString("en-US", {timeZone: "Asia/Tokyo"});

Then you can save the value to a Custom variable and use it.

Note: You can change the timezone to any timezone you want. Here's the list of standard timezone names:

https://en.wikipedia.org/wiki/List_of_tz_database_time_zones