Pick whatever reference point you prefer and stick with it. Depending on why you're keeping track of it, different references might make sense.
I tend to track the bottling date, for no other reason than it's a convenient time to mark the box. If I wanted to mark the brew date, I'd have to look that up, so I just stick a label with today's date on the box and file it away in the closet to condition. It also makes sense because that's the start of carbonation, so I need to know that date to decide when to start checking whether it's done.
I have a log entry in my brewing log that indicates when I bottled it, so I can use that to find the brew notes if I need to.