Question Game Mechanic longer ench

Quote

Increased or decreased durations are rounded to a whole number of seconds. Testing reveals that the rounding method for most effects is half to even.

Duration - Guild Wars Wiki (GWW)

wiki.guildwars.com

For your example: 15 * 1.18 = 17.7, which rounds to 18.

Duration rounding is quite complicated, there is no easy single rule that predicts all cases. The wiki is wrong/incomplete on this subject (surprise surprise...)

What's definitely true:
- Effect durations are rounded to whole seconds
- Casting time durations are rounded to smaller than seconds (probably milliseconds)

For the rest, the rounding seems to depend on the source of modification. Rounding can be:
- always up
- always down
- half to even (which makes no sense at all but that's a different discussion)
- any of the above with the added requirement of at least 1 second difference
- additional rules when the result is 0 or 1 second

For example, let's compare a +20% enchanting weapon mod and the skill Blessed Aura at 6 DF also giving +20%. If you cast a 1-second enchant with Blessed Aura, the result is 1 s duration. But if you cast it with an enchanting mod, the result is 2 s. The enchanting mod has the requirement of at least adding 1 s to the original duration.

To complicate things further, when multiple sources are stacked, sometimes only the end result is rounded, and sometimes rounding is applied in between calculation as well.

The famous example of this is the infinite enchantment duration exploit, which works because the result of the Air effect is rounded before Mark is applied.

So in my experience, it's best to test each source in game, which fortunately is easy to do in most cases.