In columns A and B you must have dates, and in row 1 of D to N you must have the year, example:
<table border="1" cellspacing="0" style="font-family:Calibri,Arial; font-size:11pt; background-color:#ffffff; "> <colgroup><col style="font-weight:bold; width:30px; " /><col style="width:109.31px;" /><col style="width:121.66px;" /><col style="width:76.04px;" /><col style="width:42.77px;" /><col style="width:42.77px;" /><col style="width:42.77px;" /><col style="width:42.77px;" /><col style="width:42.77px;" /><col style="width:42.77px;" /><col style="width:42.77px;" /><col style="width:42.77px;" /><col style="width:42.77px;" /><col style="width:42.77px;" /><col style="width:42.77px;" /></colgroup><tr style="background-color:#cacaca; text-align:center; font-weight:bold; font-size:8pt; "><td > </td><td >A</td><td >B</td><td >C</td><td >D</td><td >E</td><td >F</td><td >G</td><td >H</td><td >I</td><td >J</td><td >K</td><td >L</td><td >M</td><td >N</td></tr><tr style="height:19px ;" ><td style="font-size:8pt; background-color:#cacaca; text-align:center; " >1</td><td style="background-color:#ffff00; text-align:center; ">End</td><td style="background-color:#ffff00; text-align:center; ">Start</td><td style="background-color:#ffff00; text-align:center; ">Months</td><td style="background-color:#ffff00; text-align:center; ">2011</td><td style="background-color:#ffff00; text-align:center; ">2012</td><td style="background-color:#ffff00; text-align:center; ">2013</td><td style="background-color:#ffff00; text-align:center; ">2014</td><td style="background-color:#ffff00; text-align:center; ">2015</td><td style="background-color:#ffff00; text-align:center; ">2016</td><td style="background-color:#ffff00; text-align:center; ">2017</td><td style="background-color:#ffff00; text-align:center; ">2018</td><td style="background-color:#ffff00; text-align:center; ">2019</td><td style="background-color:#ffff00; text-align:center; ">2020</td><td style="background-color:#ffff00; text-align:center; ">2021</td></tr><tr style="height:19px ;" ><td style="font-size:8pt; background-color:#cacaca; text-align:center; " >2</td><td style="text-align:right; ">01-ago-2019</td><td style="text-align:right; ">01-nov-2018</td><td style="text-align:right; ">10</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">2</td><td style="text-align:right; ">8</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td></tr><tr style="height:19px ;" ><td style="font-size:8pt; background-color:#cacaca; text-align:center; " >3</td><td style="text-align:right; ">01-mar-2019</td><td style="text-align:right; ">01-may-2018</td><td style="text-align:right; ">11</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">8</td><td style="text-align:right; ">3</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td></tr><tr style="height:19px ;" ><td style="font-size:8pt; background-color:#cacaca; text-align:center; " >4</td><td style="text-align:right; ">05-may-2019</td><td style="text-align:right; ">01-ene-2019</td><td style="text-align:right; ">5</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td><td style="text-align:right; ">5</td><td style="text-align:right; ">0</td><td style="text-align:right; ">0</td></tr></table><br /><table style="font-family:Arial; font-size:10pt; border-style: groove ;border-color:#00ff00;background-color:#fffcf9; color:#000000; "><tr><td ><b></b></td></tr><tr><td ><table border = "1" cellspacing="0" cellpadding="2" style="font-family:Arial; font-size:9pt;"><tr style="background-color:#cacaca; font-size:10pt;"><td >Cell</td><td >Formula</td></tr><tr><td >D2</td><td >=IFERROR(DATEDIF(MAX($B2,DATE(D$1-1,12,31)),MIN($A2,DATE(D$1,12,31)),"m")+1,0)</td></tr></table></td></tr></table>