Time allows and facilitates duality to operate in our sphere of existence to the extent that it does.  While one can feel joy and anger at the same time, that normally does not happen.  Circumstances, occurring through time, produce joy.  And other circumstances, occurring through time, produce anger.

The consecutive movement of time, going from one moment to the next, is by nature dualistic.  First the one moment exists, and then the second moment exists.  There is a spectrum of past, present and future that underlies time, as there is a feeling spectrum from joy to anger, or a physical spectrum from hot to cold.  All of these spectrums are dualistic, but the spectrum of time allows the other spectrums to be experienced to their fullest.  Time is like machinery producing the fertile ground for dualism to exist, while being dualistic itself.