Where the line falls
On an import: the container discharges and free time starts. Every day it remains in the terminal past that allowance accrues demurrage (the box is occupying port real estate). Once gated out, a second clock runs: every day the consignee keeps the carrier's container for unpacking beyond equipment free time accrues detention (they are borrowing the line's asset). Free time is typically 4–7 calendar days each, but it is carrier-, port-, and contract-specific, and D&D tariffs escalate steeply in tiers.
The charges compound with each other and with storage: a container stuck awaiting documents can rack up terminal demurrage while the clock toward detention hasn't even started. On disputed cargo, D&D routinely exceeds the ocean freight itself.
Why D&D happens — and the desk-level fixes
Most D&D is not caused by trucking shortages; it is caused by paperwork latency. The classic chain: originals arrive late (courier), or the SI was missed so the telex is delayed, or duty payment waits on an invoice query — and the container sits through its free time while emails go back and forth. The fixes are unglamorous and effective: choose release mechanics that match payment terms (telex or seaway instead of couriered originals), file customs pre-arrival where the regime allows, book delivery slots against free-time expiry rather than vessel arrival, and track every container's free-time clock explicitly instead of discovering it on the carrier's invoice.