ILMT within 90 days: the rule that voids sub-capacity.

Sub-capacity licensing is the difference between paying for the cores your IBM software actually uses and paying for every core in the physical host. The gap is large. WebSphere using four of a host's thirty two cores is 480 PVU under sub-capacity and 3,840 PVU at full-capacity, an eight fold difference on a single server. That benefit is not automatic. It is conditional, and the first condition is the 90 day rule.

What the rule says

To claim sub-capacity, you must deploy an approved measurement tool within 90 days of the first eligible sub-capacity deployment. For most environments that tool is the IBM License Metric Tool, ILMT, though IBM also recognizes approved alternatives such as Flexera One ITAM and HCL BigFix Inventory. Deploy it inside the window and the sub-capacity clock starts cleanly. Miss the window and IBM is entitled to charge full physical capacity for the period that was not measured.

Why the deadline is missed so often

The 90 days runs from first eligible deployment, not from the day someone remembers ILMT. A team stands up a new WebSphere or Db2 instance to hit a project date, the measurement tool is treated as a later task, and the window quietly closes while the software runs in production. By the time anyone checks, the period is already exposed, and no amount of later tooling retroactively measures a quarter that was never instrumented.

• The 90 days starts at first eligible deployment, not at go live of ILMT
• An approved tool is required: ILMT, Flexera One ITAM, or HCL BigFix Inventory
• A late deployment does not retroactively cover the unmeasured period
• Full-capacity charging applies to the gap, host by host

Deployment is necessary but not sufficient

Hitting the deadline is only the first of several conditions. The tool must then run continuously, and you must generate and retain quarterly reports for two years. ILMT also miscategorizes installs and needs manual correction, so a tool that is technically present but misconfigured or unreviewed can still fail to support the sub-capacity claim. The 90 day rule opens the door; continuous, accurate operation keeps it open.

What to do if the window was missed

A missed deadline is a defendable position, not an automatic loss. The exposure is bounded by the unmeasured period, and the correct response is to deploy and stabilize the tool now, document the timeline precisely, and reconcile what can be evidenced against what IBM is claiming. In an audit, the difference between a full-capacity default and a negotiated outcome is the quality of that timeline and the evidence behind it.