Calendar-aligned SLOs
Engineering teams typically implement service level objectives (SLOs) using rolling time windows for operational purposes, as they provide ongoing visibility into service health and align well with agile sprint workflows. However, when it comes to management reporting, compliance verification, and SLA tracking, calendar-aligned SLOs offer significant advantages.
Calendar-aligned SLOs reflect service reliability based on specific calendar periods (months, quarters, or years), providing clear reporting boundaries that match business cycles and contractual obligations. This approach eliminates the need to extrapolate data from rolling windows, which can lead to inaccurate representations of your service's true uptime.
This guide explores when to use calendar-aligned SLOs, how they complement rolling time windows, and provides a real-world implementation case study to help you implement effective reliability reporting for different stakeholders.
In a nutshell
Calendar-aligned time windows are defined by fixed start and end dates. When you configure an SLO with a calendar-aligned time window, you specify its duration and start date. After the SLO is saved, the time window anchors to this start date, collects data throughout the period, and, at the calendar boundary, discards data from the previous interval and begins a new period with reliability values restored to 100%.
| Example start and end dates | Frequency | Next time window start date |
|---|---|---|
| April 15, 2025 – April 14, 2026 | Yearly | April 15, 2026 |
| April 15 – July 14 | Quarterly | July 15 |
| April 15 – May 14 | Monthly | May 15 |
| April 15 – April 21 | Weekly | April 22 |
| April 15, 15:00:00 – April 16, 14:59:59 | Daily | April 16, 15:00:00 |
This example shows how different frequencies of calendar-aligned time windows work. When configuring your SLO, you can set the duration ranging from days to quarters, with a maximum period of 366 days. For example, you can configure a time window that repeats every 19 days or 7 weeks—whatever best fits your needs.
The calendar-aligned time window concept is straightforward in standard cases, when a time window starts on a day that exists in every month (1–28), the next time window will predictably start on the same day.
However, for time windows starting on days 29–31, complications arise when the next time window doesn't have these dates. In such cases, Nobl9 normalizes the date to ensure the next time window starts on a valid date. Below are the examples of date normalization.
| Time window start | Increment | Next time window start | Explanation |
|---|---|---|---|
| January 30 non-leap year | +1 month | March 2 | February doesn't have 29th day |
| January 30 leap year | +1 month | March 1 | February has 29 days |
| February 29 leap year | +1 year | March 1 following year | Since the following February ends on the 28th, the date is normalized |
Practical use
What goals do calendar-aligned time windows serve best?
Observations show that, typically, calendar-aligned SLOs are beneficial for providing a broader view of system performance over reporting periods. This type of time window gives executive, customer support, and procurement teams essential information for planning, compliance reporting, and holding vendors accountable to service-level agreements.
| Goal | Business domain | Practical use |
|---|---|---|
| SLA compliance | Management | Clear and accessible uptime reports for contractual obligations with vendors or customers to ensure SLA targets are met |
| Performance review | Management | Organizational goals and metrics tracking in a clear format |
| Project planning | Management | Resource allocation based on reliability metrics over calendar periods |
| Tracking customer impact | Customer support | Straightforward explanation of SLA and performance to communicate to customers during reporting cycles |
| Incident reporting | Customer support | Provides clarity when outages occur bu aligning with defined reporting periods |
| Compliance reporting | Executive | Bridge the gap between technical results and business obligations by tying SLOs to fiscal or contractual periods |
| Stakeholder communication | Executive | Deliver transparent reliability metrics directly tied to quarterly or annual performance goals |
| Vendor accountability | Procurement | System reliability tracking to assess vendor adherence to SLAs and drive renewal, replacement, or renegotiation discussions |
Case study: are we achieving our uptime goals?
An organization operating a large e-commerce platform needed to create management reports using their existing SLOs. They used SLOs to ensure critical user journeys, such as catalog display or checkout, remained reliable for their customers.
Although they had rolling time-window SLOs in place, they found answering management's questions about overall uptime challenging, as rolling windows required additional data conversion to align with calendar periods.
Solution with calendar-aligned time windows
To simplify their setup, the organization configured calendar-aligned SLOs using their existing SLI queries, making reporting unambiguous.
As error budget alerts were already managed through rolling time window SLOs, no additional alerting policies were necessary for the new calendar-aligned SLOs. This configuration allowed teams to deliver up-to-the-minute uptime reports without the need for manual data transformation or extra reporting steps.
The combination of both time window types gave engineering, operations, and application development teams proactive alerts to emerging issues, while management and customer support could track service performance with precision for any given month, quarter, or year.
End-result
Before implementing SLOs with the calendar-aligned time window, the organization found it difficult to determine their compliance with SLAs and often reacted to incidents after the fact. With these changes, they gained clear, continuous insight into system health, supporting data-driven decisions related to contractual agreements, user experience, and development velocity.
