I am saving a counter number in user storage. I want to provide some content to the user which changes daily using this counter. So every time the counter increases by 1 the content will change. The problem is the timezone difference. Is there anyway to run a function, daily which will increase this counter by 1. I could use setInterval() which is a part of the NodeJs library but that won't be an accurate "daily" update for all users.
User storage is only available to you as a developer when the Action is active. This data is not available once the Action is closed, so you wouldn't be able to asynchronously update the field. If you do want asynchronous access, I'd suggest using an external database and only storing the database row key in the user's userStorage. That way you can access the data and modify it whenever you want.
The setInterval method will run a function periodically, but may not work in the way you want. It only runs the function while the runtime is active. A lot of services will shut down a runtime after a period. Cloud Functions, for example, run sometimes but then will shut down when not used. Additonally, Cloud Functions can be run several times in parallel instances, executing a setInterval function several times in parallel. That would increment the counter more times than you want.
Using a dedicated Cron service would help reduce the number of simultaneous executions while also ensuring it runs when you want.
You are unable to directly access the user's timezone within the Action, meaning you won't be able to determine the end of a day. You can get the content to change every day, but it'll have some sort of offset. To get around this, you could have several cron jobs which run for different segments of users.
Using the conv.user.locale field, you can derive their language. en-US is generally going to be for American users, which generally are going to live in the US. While this could result in an odd behavior for traveling, you can then bucket users into a particular period of execution. Running the task overnight, either 1AM or 4AM they'll probably be unaware but know that it updates overnight.
You could use the location helper to get the user's location more precisely. This may be a bit unnecessary, but you could use that value to determine their timezone and then derive that user's "midnight" to put in the correct Cron bucket.