Part 2 ยท Windows
Time matters as much as target

SLO windows and the math of nines

The target is incomplete until you also define the measurement window.

Rolling 28 days

  • Exactly four weeks, so weekday patterns stay balanced.
  • Smoother than calendar-month resets.
  • Usually a better operating window for engineering decisions.

Calendar month

  • Aligns with reporting and billing.
  • Can hide incidents near the reset boundary.
  • Often simpler for business summaries than for operations.
Every extra nine tightens the target. The window decides how that pressure is felt.

That is why SLO definition is both math and operating design.

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