The summit doesn't set your turnaround time. Sunset does.

Quick answer: Work backward from sunset the night before and write down one turnaround time. When the clock hits it, you turn around, summit or no summit. Carry traction and a headlamp even if the trail looks packed.

Start at sunset and count backward

In December, sunset across the northern states can arrive before 4:30 p.m. Look up the exact time for your trailhead. Don't guess, and don't use the time for the nearest big city.

Now subtract. Say sunset is 4:35. You want to be off the trail, or at least on easy ground, by about 4:00, so you'd have a little light left over. If the descent will take two hours, you need to be turning around by 2:00 at the latest. If the summit is still a mile away at 2:00, it isn't your summit today.

That's the whole system. One time, written on your hand or in your phone's lock screen note, chosen when you were warm and thinking clearly.

Most people do it the other way around. They pick a peak, estimate the climb, and hope the daylight works out. Winter punishes hope. A bad day in July means a late dinner. A bad day in January means a cold night, because the temperature falls hard once the sun is gone.

Winter pace is slower than you think

The old rule of thumb for planning is about two miles an hour on flat ground, plus an hour for every 2,000 feet you climb. Treat that as a summer number. Snow, ice, a heavier pack and stopping to adjust gear all eat into it. Plan on being noticeably slower, and be honest after your first mile about how slow you are. If you're behind your own schedule early, shorten the route right then. Don't wait to see if you catch up. You won't.

Descending isn't free either. Going down an icy trail is often slower than going up it, because you're placing every foot like you're carrying eggs.

Match your traction to the trail

Boots alone are fine on dry dirt. They aren't fine on a packed trail that's turned to ice, and that's what most popular winter trails become after a few hundred people have walked them.

• Microspikes are a chain-and-spike harness that stretches over your boot. Kahtoola MICROspikes are the best-known. They're the right answer for packed snow and ice on ordinary trails, and for most winter day hikes they're all you need. Take them off to cross bare rock or pavement. Metal on rock is slippery, and it chews up the spikes.
• Snowshoes are for deep, unpacked snow, where you'd otherwise sink to your shins or knees on every step. Sinking like that is called postholing. It's exhausting, it wrecks your pace, and it tears up the trail for the next person.
• Crampons and an ice axe are for steep, hard snow and real mountaineering. They also take real instruction. Buying them doesn't make you qualified to use them. If the route needs them, take a class or go with someone who knows.

Trekking poles help in all three cases. Pack your traction at the trailhead and put it on at the first sign of ice. It takes two minutes. A slip on a steep, frozen slope takes much longer than that to recover from.

What goes in the pack

The basics haven't changed, and the ten essentials still apply. In winter a few of them matter more.

A headlamp, always. Not your phone light. Put fresh batteries in the night before and carry a spare set. Cold drains batteries fast, so keep the spares in an inside pocket against your body.

An extra insulating layer. Bring a puffy jacket you won't wear until you stop. Sweat is the enemy. When you stop moving, damp clothes pull heat off you quickly. Dress so you're slightly cool at the start of the climb, and peel layers before you're soaked. Skip cotton entirely. Wool or synthetic only.

Food you can eat cold. A frozen energy bar is a brick. Keep snacks in a pocket close to you, and eat small amounts often.

Water that won't freeze. A hard-sided bottle stored upside down in an insulated sleeve is a reasonable fix, since ice forms at the top first and the cap stays free. A hot drink in a vacuum bottle is worth its weight, and it's a morale boost when you've reached your turnaround spot and turned your back on the view.

Then add the paper side. Before you leave, tell somebody at home where you're going, what trail, when you'll be back, and when to start worrying. Your wife shouldn't be left guessing at 6 p.m. Give her the turnaround time too. It tells her what to expect.

The hard part is turning around

You'll want to push on. You drove two hours. The summit looks close. Your buddy says, "We're almost there." That's the moment the plan earns its keep.

Be honest about the trade-off. Sometimes you'll turn around and the weather would have held and the light would have been fine. You'll feel a bit foolish. That's the price of a rule that works every time instead of most of the time.

If you're hiking with others, say the turnaround time out loud at the trailhead so nobody has to be the one to bring it up later. A good leader isn't the one who gets the group to the top. It's the one who gets everyone back to the cars with all their toes. Hold that line even when someone gets short with you about it.

Know the other signs too. Clouds building, wind picking up, snow starting when none was forecast, or someone in your group getting quiet and shivering. Any of those is a reason to turn back early. The weather that kills hikers covers how to read it before it reads you. And if the trail disappears and you can't say for sure where you are, stop and use the first hour after you realize you're lost. Don't keep walking and hope.

Leave the trail better than you found it

Winter is hard on trails, and the damage is easy to avoid. Walk in the middle of the packed track, even through slush. Stepping around it widens the trail and tramples what's growing beside it. Don't postholing along the edge. Pack out everything you carried in, including orange peels and the wrapper from that frozen bar. If you find someone else's glove or granola bag on the way down, carry it out too. It costs you nothing.

Get back to the truck with daylight to spare, boots off, hot drink in hand, and you've done it right. The mountain will be there next Saturday. Make sure you are too.