Why the date is rarely simple
Each year of tax has its own clock, so a taxpayer with several years of balances has several different expiry dates. The clock also pauses in defined circumstances, including while an Offer in Compromise is pending, during a collection due process hearing, through a bankruptcy and for a period afterwards, and while you are outside the country for an extended time.
Every one of those pauses pushes the date out, and they accumulate. This is why the expiry date shown on an IRS transcript needs interpreting rather than reading.

