wa tilka'l ayyamu nudawiluha bayn an-nas. and these are days (of varying fortune) that we give to men by turns. --- from imam razi's tafsir: qaffal said 'mudawalah' means change of a thing from one state to another. it is said, 'tadawalat-hu'l aydi' / changed hands, that is when it is transferred from one hand to another. like it is said in the verse: kay la yakuna dulatan bayna'l aghniya'yi minkum. [hashr, v.7] / so that it [charity] does not merely change hands between the wealthy among you. that is, you will merely exchange it [between the rich] and not share it with the poor and needy. it is said 'the world is transitionary' because it [favor and fortune] changes from a people to another; and then from them to yet others. it is said: 'time has changed him in such a manner', when a person transits towards it. the meaning is: that the [fortune in the] days in this world are interchanging, shifting amongst people such that circumstances are not everlasting - whether good fortune or adversity. on a day, one might find happiness while his enemy is in affliction; and another day it might be completely opposite. nothing remains permanently in its state nor does its influence or impact abide forever. -- Allah ta'ala knows best.