subReddit means a dedicated forum or community to any topic or users interest on Reddit website, where joined users only discuss and share their experience over specific topic only.
Basically subReddits are denoted by /r/ symbol, following the subReddit name such as /r/technology or /r/networking.
Throughout the phase when you are a Reddit user, you join many subReddits pages as per you interest.
How to See When You Joined a subReddit
There isn’t any direct way to go into specific subReddit and check when you joined it. you can try out the alternate ways to know the joining date for a subReddit.
Method 1: Search and go to the First Comment
You can try to go back into history and check out when you firstly did the comment or any activity in that particular subReddit. It would provide you an idea that when you joined that particular subReddit.
Method 2: Ask subReddit Admin to provide the Detail
You can try to reach out that subReddit admin to get the information when you joined that particular group if it is important to know.
subReddit Admins have the whole information access for users activities and can help you out to provide the joining date.
Method 3: Reach out to Reddit using Data Request Form
You can also get the joined SubReddits and other details from Reddit itself by filling the data request form https://www.reddit.com/settings/data-request.
You need to select out all details which you want to collect for you Reddit account and submit the form.
Reddit typically takes somewhere around 30 days to prepare the data and send you details over your personal registered mail id.