Los Angeles Lawyers Know Anything About Statute Of Limitations On Late Rent Fees?

Los Angeles landlord sent a debt collection letter for late fees on rental payments (one from 2016, one from 2017, and one from 2018). Because of the time passed, we can’t prove these payments were made on time, and we were only made aware of this last week. The letter included a ledger of charges, but the line items and amounts do not match what the tenant payment app say- the app also lists more recent erroneous charges that I do have proof of payment on. Landlord is just saying it’s all late fees. I’m wondering A how long after a late rent fee can a landlord legally pursue repayment? And B, if I do have to pay, how I can ensure accurate documentation on the app for my receipts to protect myself in the future? Thank you for any insight and advice!