In[]:=
index={103.244,145.125,156.575,179.2,225.758,315.217,235.025,248.392,272.283,265.067,259.2,265.517,259.2,254.6,303.833,353.425,366.775,339.092};time={1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18};trainData=Thread[time->index];p=Predict[trainData,Method"NeuralNetwork"];p[{19,20,21}]
Out[]=
{324.518,313.069,301.056}