Which software platform are you using for management? Many platforms have several note fields in unit setup that allow you to attach a note to each specific unit. This would be a good place to record the date of replacement, and then in the future you can run a report that lists that data field, and you could sort it in Excel to find the oldest, newest, etc.
It's also a good place to record model and serial numbers of the appliances. As a property ages it will often end up with several different appliance models in use, and having this info on file helps streamline the repair process for maintenance staff.
We use OneSite. When entering invoices for appliances, carpet, etc. there is a field for apartment number. You can pull a unit costs ledger report that will show all expenses for each unit.
If you are tracking appliance replacements, I’d recommend that you record the appliance type, appliance model, serial number, warranty, supplier and invoice info, warranty claims, pictures, notes about conditions, major repairs, etc. As you can tell, it’s tricky keeping all of this information up-to-date since it involves the initial purchase and any notable events.
My recommendation is to use a property maintenance software platform that allows you to track all of this information when you purchase, inspect or process work orders on your appliances. Just be aware that most systems let you track items purchased, but they don't help you with the rest of the process. As Matt mentioned, you can add notes into your property management system, but your staff will get better and more timely information if they use a software platform that records this information as a part of the purchasing, inspection and work order process.
Build an Excel spreadsheet. It may take time to add appliance information but it'll be worth it. We have spreadsheets for carpet/vinyl, appliances, registered vehicle tags, wall color, etc. They come in handy for everything.
My portfolio uses OneSite and we utilize the Asset section of the Facilities platform, which tracks all applicances by serial number and apartment, you can transfer the asset if you move it or retire it when you replace it. It will alert you if you enter a service request on an appliance that is currently under warranty.