> Why billing systems are a nightmare for engineers My guess: Because basically nobody wants to develop one of these as a hobby FOSS project. The whole idea is

> Why billing systems are a nightmare for engineers My guess: Because basically nobody wants to develop one of these as a hobby FOSS project. The whole idea is collecting people's money, which when you write your own code for public benefit, you basically don't do :-) > We quickly decided to add plans, and 'pay-as-you-go' features So, the author talks about things getting more complex, beyond their expectations. But - why did they not expect this? I mean, they know that companies pricing changes over time; that payment models change, including rates, cadence, use of internal credit etc. We all know this without ever having worked on a billing system. > Qonto is a 'neobank' ... Fintech unicorn So they were expecting billing to be easy for a bank-like endeavor? Hmm. > #1 - Dates The date problems per se are quite mundane and any non-trivial software system which involves timed activity deals with them. The billion period problems are not really date problems and are a recap of what OP had already written about. They are also problems most of us are well aware of from our personal experience as people in modern society ("Does my bus pass expire a day after I bought it, i.e. 24 hours later, or does it expire at the turn of the day = at midnight?") etc.