j2y.dev — Today I LearnedShort notes on things Alexandre Joly discovers while building software.https://j2y.dev/enAny 2nd pillar affiliation in a tax year caps your pillar 3a deduction at CHF 7'056https://j2y.dev/til/pillar-3a-pk-affiliation-kills-large-deduction/https://j2y.dev/til/pillar-3a-pk-affiliation-kills-large-deduction/If you're self-employed without a pension fund, you get the large 3a deduction (20% of income). But any PK affiliation during the year — even a single month as a side employee — kills it.Wed, 01 Apr 2026 00:00:00 GMTTime series models will silently overfit if you use random train/test splitshttps://j2y.dev/til/xgboost-time-series-leak/https://j2y.dev/til/xgboost-time-series-leak/Why shuffled train/test splits cause silent data leakage in time series models, and how TimeSeriesSplit fixes it.Sun, 08 Mar 2026 00:00:00 GMTToday I hired a CFO - an n8n workflow to automate my invoices.https://j2y.dev/til/today-i-hired-a-cfo/https://j2y.dev/til/today-i-hired-a-cfo/How I automated monthly invoicing with n8n, Harvest, and Beancount — saving 15 minutes/month at the cost of 2 days engineering. Classic.Fri, 19 Dec 2025 00:00:00 GMTSimple Snippet to Add a Help Description to a Bash Scripthttps://j2y.dev/til/bash-script-help/https://j2y.dev/til/bash-script-help/A reusable bash boilerplate that auto-generates --help output from inline comments using awk. Zero external dependencies.Sun, 26 Oct 2025 00:00:00 GMTSelf-Signed Certificate Authority Creation with OpenSSLhttps://j2y.dev/til/self-signed-certificate-authority-creation-with-open-ssl/https://j2y.dev/til/self-signed-certificate-authority-creation-with-open-ssl/Step-by-step guide to creating a self-signed certificate authority with OpenSSL for homelab Kubernetes clusters and local HTTPS.Mon, 23 Jun 2025 00:00:00 GMTModule Federation shared dependencies need explicit version constraintshttps://j2y.dev/til/module-federation-shared-deps/https://j2y.dev/til/module-federation-shared-deps/Webpack Module Federation can silently ship duplicate dependencies without version constraints — how to configure shared deps correctly.Fri, 15 Nov 2024 00:00:00 GMTLonghorn requires a node selector when mixing storage and non-storage nodeshttps://j2y.dev/til/k3s-longhorn-node-selector/https://j2y.dev/til/k3s-longhorn-node-selector/Longhorn storage will try to use all nodes by default — here's the node selector config to restrict it to designated storage nodes.Tue, 03 Sep 2024 00:00:00 GMT