var vel = 100;
var pInit = position.valueAtTime(0)[0]; // initial position of a cloud
var tInit = (thisComp.width-pInit)/vel; // time it takes to cross the right border starting from init position
var tFull = (thisComp.width+width)/vel; // time it takes to cross the frame starting behind the left border
var offset = 0;
tBound = tInit;

if(time > tBound) {
  tBound += tFull;
  offset += thisComp.width+width;
}

[value[0]+time*vel-offset,value[1]]