Laytime vs time on demurrage: what's the difference
Every laytime dispute starts with someone assuming the two clocks are the same clock. They aren't. Laytime is the time the charter party allows for loading or discharging. Time on demurrage is what happens after that allowance runs out and the vessel is still sitting there, waiting to load or waiting to discharge, burning hours that now cost the charterer money at the demurrage rate.
Get the distinction wrong and you get the wrong number on the final statement of facts. Get it right and you know, days before the agent's telex confirms it, roughly where you stand.
What laytime actually covers
Laytime is the window set out in the charter party, usually expressed as a fixed number of running days or as weather working days (SHINC or SHEX, depending on what was negotiated). It starts ticking once notice of readiness is tendered and accepted, not necessarily the moment the vessel physically arrives. That gap between "arrived" and "laytime commenced" is where most of the arguments live, because NOR acceptance depends on terms like WIPON, WIBON, or WIFPON that decide whether a vessel sitting at anchor counts as having tendered valid notice at all.
Once laytime starts, the clock runs against the cargo operation, loading or discharging, regardless of the ship's schedule. Sundays, holidays, and bad weather may or may not count, again depending on what the charter party says. This is the part that gets mis-stated more than any other clause in a fixture: two charter parties with nearly identical cargo and nearly identical ports can produce very different laytime totals because one excludes Fridays and the other doesn't.
Laytime calculation under the charter party is, in practice, an exercise in reading the exceptions clause more carefully than the main laytime clause. The headline number, say 72 hours SHINC, tells you almost nothing until you know what stops the clock.
When laytime expires, demurrage starts
Mechanically, time on demurrage begins the instant laytime is used up, whether or not loading or discharging has finished. There's no grace period and no need for a separate notice. The vessel doesn't have to do anything differently. The charterer's exposure just starts accruing at the demurrage rate instead of at zero.
Legally, the rule charterers get burned by is "once on demurrage, always on demurrage." Once that clock starts, the exceptions that paused laytime (weekends, weather, strikes) generally stop applying unless the charter party expressly says they continue to apply during demurrage too. A clause that excludes Sundays from laytime does not, on its own, exclude Sundays from demurrage. If you want that protection, it has to be drafted in. This single point has decided more demurrage arbitrations than almost anything else in the fixture.
Where the charterer actually loses the thread
None of this is complicated on paper. It gets complicated in practice because the information a charterer needs, when NOR was tendered, when the berth actually opened up, how long the vessel sat at anchor before any of that happened, comes through agents, port logs, and telex updates that lag the real situation by days. A vessel can be sitting in a queue outside a congested port for a week before that shows up anywhere a charterer can see it, and by the time it does, the laytime clock may already be most of the way to demurrage.
That lag is the actual commercial risk, not the clause wording. Knowing the clause distinction matters, but knowing how many vessels are stacked up outside the discharge port on any given day, independent of what the agent reports, is what lets a charterer see demurrage exposure building before the statement of facts confirms it. A daily count of vessels queued at anchor, tracked over time, gives you that earlier read.
If you're working a fixture into a congested port right now, that's exactly the gap Port Congestion Watch is built to close.