Five year diaries are nothing new - I actually had one as a teenager. I didn't know what to write or where to start and unsurprisingly lost momentum (and interest) quite quickly. I know that won't happen this time round; yes, because I'm older but also because the diary presents you with a 'concept' which will (if you stick with it) basically make you want to keep writing.
The idea is simple: you write one sentence every night - it can be anything from your day that you want to remember. Then as the years go by, you build up your diary and can start comparing entries so you can see what you were doing or feeling on the same day in previous years.
There's no years marked so you could actually buy the diary and start at any point in the year, on any year you like. My sister owns the previous diary by the Potter Style range which was a Q&A journal - a much better idea in my opinion because the diary asked you a new question every day and you would be able to compare 5 years of answers to the same question on each day. More fun but that diary is sadly nowhere to be found and this one is a decent substitute: you get a quote/thought for each day - your diary entries don't have to relate to the quote though so your entries could be anything at all. Obviously if you don't want the quotes each day or you don't like the blue & yellow quirky design, you could buy yourself any 5-year diary that takes your fancy and keep your 5 year record like that.
Either way, it's a great idea and it's amazing how one sentence (or a little cluster of sentences as I often make!) can prompt you to remember the day with such vivd recollection in years to come. It's also perfect for people who like the idea of keeping a memory journal or diary of some sort but just don't have the time!