Naismiths rule, in the usual form, is one hour for every 3 miles plus one hour for every 2,000 feet of ascent. Twelve miles and 2,000 feet is five hours. Descent is not in the original rule. A steep descent still costs time, so this app adds some of it back when the grade is steeper than about 10 percent.
Your group may not walk at 3 miles an hour. Casual is 2 mph and 800 feet of climb per hour. Steady is 2.5 mph and 1,000. Fast is 3 mph and 1,500. You can type your own pair. The segment time is the slower of that Naismith estimate and a Tobler pace on the same grade. Tobler is quickest on a gentle downhill, about a 5 percent drop, and slower as the hill gets steeper.
The free result is the total. $5 shows Naismith next to the segment clocks.
Turnaround is a pace sketch from Naismith and Tobler, not a guide and not a weather report. Leave earlier for storms, a slow group, or a route you do not know. A headlamp does not make a late return safe.