Time on Brian Faust

Global State Makes Local Reasoning Impossible

Sat, 01 Aug 2020 00:00:00 +0000

A repository test created a shipment two days before now, then counted shipments between the start and end of the current month. It passed most days. During the first two days of a month, “two days ago” belonged to the previous month and the count became zero.

Nothing in the test name mentioned a calendar boundary. Nothing in the method signature mentioned time. The same test logic produced a different result because its generated data moved with one more input: the instant at which the test ran.

Time Zones Are Part of the Data Model

Thu, 01 Jun 2017 00:00:00 +0000

One metering path assigns an event to a billing month after converting its occurrence time into the account’s timezone. The same path derives its daily usage key directly from the occurrence timestamp without that conversion.

Near midnight, those two fields can disagree:

event instant: May 31, 21:30 UTC
local business time: June 1, 00:30
billing period: June
stored usage date: May 31

Each expression looks like a date calculation. They model different things. The defect appears only when the difference changes a boundary.