Try the following formula, and copy down (note in my example, it is in mm/dd/yyyy format due to my system locale)
<b>Excel 2007</b><table cellpadding="2.5px" rules="all" style=";background-color: #FFFFFF;border: 1px solid;border-collapse: collapse; border-color: #A6AAB6"><colgroup><col width="25px" style="background-color: #E0E0F0" /><col /><col /></colgroup><thead><tr style=" background-color: #E0E0F0;text-align: center;color: #161120"><th></th><th>A</th><th>B</th></tr></thead><tbody><tr ><td style="color: #161120;text-align: center;">1</td><td style="text-align: right;;">1/1/2010</td><td style="text-align: right;;">1/1/2012</td></tr><tr ><td style="color: #161120;text-align: center;">2</td><td style="text-align: right;;">6/1/2008</td><td style="text-align: right;;">6/1/2011</td></tr><tr ><td style="color: #161120;text-align: center;">3</td><td style="text-align: right;;">6/26/2011</td><td style="text-align: right;;">6/26/2011</td></tr><tr ><td style="color: #161120;text-align: center;">4</td><td style="text-align: right;;">6/1/2010</td><td style="text-align: right;;">6/1/2011</td></tr><tr ><td style="color: #161120;text-align: center;">5</td><td style="text-align: right;;">9/5/2010</td><td style="text-align: right;;">9/5/2011</td></tr><tr ><td style="color: #161120;text-align: center;">6</td><td style="text-align: right;;">4/5/2011</td><td style="text-align: right;;">4/5/2012</td></tr><tr ><td style="color: #161120;text-align: center;">7</td><td style="text-align: right;;">1/1/2011</td><td style="text-align: right;;">1/1/2012</td></tr></tbody></table><table cellpadding="2.5px" rules="all" style=";background-color: #FFFFFF;border: 1px solid;border-collapse: collapse; border-color: #A6AAB6"><thead><tr style="background-color: #E0E0F0;text-align: center;color: #161120"><th><b>Sheet1</b></th></tr></td></thead></table><br /><br /><table width="85%" cellpadding="2.5px" rules="all" style=";border: 2px solid black;border-collapse:collapse;padding: 0.4em;background-color: #FFFFFF" ><tr><td style="padding:6px" ><b>Worksheet Formulas</b><table cellpadding="2.5px" width="100%" rules="all" style="border: 1px solid;text-align:center;background-color: #FFFFFF;border-collapse: collapse; border-color: #A6AAB6"><thead><tr style=" background-color: #E0E0F0;color: #161120"><th width="10px">Cell</th><th style="text-align:left;padding-left:5px;">Formula</th></tr></thead><tbody><tr><th width="10px" style=" background-color: #E0E0F0;color: #161120">B1</th><td style="text-align:left">=IF(<font color="Blue">A1>DATE(<font color="Red">2011,5,19</font>),A1,IF(<font color="Red">AND(<font color="Green">A1>=DATE(<font color="Purple">2010,5,18</font>),A1<=DATE(<font color="Purple">2011,5,17</font>)</font>),DATE(<font color="Green">YEAR(<font color="Purple">A1</font>)+1,MONTH(<font color="Purple">A1</font>),DAY(<font color="Purple">A1</font>)</font>),IF(<font color="Green">MONTH(<font color="Purple">A1</font>)>5,DATE(<font color="Purple">2011,MONTH(<font color="Teal">A1</font>),DAY(<font color="Teal">A1</font>)</font>),DATE(<font color="Purple">2012,MONTH(<font color="Teal">A1</font>),DAY(<font color="Teal">A1</font>)</font>)</font>)</font>)</font>)</td></tr></tbody></table></td></tr></table><br />