Winter's Bite chill is not extended through use of runes & sigils.
I’ve noticed this too. Also on a related topic, skill tooltips need to be updated to show new stats based on modifiers like this. So if a person has sigils of chilling like this, the tool tip for the skill should show an updated chill duration to let the player know how long the chilled condition will now be.
I have recently tested this. The problem is not the sigil and rune duration, which work correctly, but the initial chill duration of Winter’s Bite.
I’ve tested this with a stop watch and that length is approximately 1.8 seconds. With +100% chill duration, the length is approximately 3.6 seconds.
Please fix this, Anet.