- Store UTC, compute day boundaries in server local timezone (new app/services/timezone.py). - summarize: fixed fee / tax credit accrue only for elapsed whole local days, integrated across contract versions — no future-day inflation, no version-switch collapse. - MQTT import_cost_total / export_revenue_total getters delegate to summarize (no inline apportionment), anchored at the active contract's earliest version effective_from so the total sensors stay monotonic and never jump backward. - effective_from: naive datetimes interpreted as server-local wall-clock, then stored UTC. - frontend ContractForm sends local-midnight naive datetime (was UTC midnight). - get_costs_summary default window uses server-local "today".