Please read the following table in HTML format from a webpage and then answer the question in detail according to the table. Each table cell is defined by a <td> and a </td> tag. Each table row starts with a <tr> and ends with a </tr> tag. 'rowspan=m' and 'colspan=n' represents a cell spaning over 'm' rows and 'n' columns.

Webpage title:
Convoys HX 229/SC 122
Table title:
Allied ships
Table:
<table id='hittask' contenteditable='False'>
<tr> <th colspan=1 rowspan=1 > Date </th><th colspan=1 rowspan=1 > Name </th><th colspan=1 rowspan=1 > Flag </th><th colspan=1 rowspan=1 > Casualties </th><th colspan=1 rowspan=1 > Tonnage (GRT) </th><th colspan=1 rowspan=1 > Sunk by </th></tr>
<tr> <td colspan=1 rowspan=1 > 16 March 1943 </td><td colspan=1 rowspan=1 > Elin K </td><td colspan=1 rowspan=1 > Norway </td><td colspan=1 rowspan=1 > 0 </td><td colspan=1 rowspan=1 > 5,214 </td><td colspan=1 rowspan=1 > U-603 </td></tr>
<tr> <td colspan=1 rowspan=1 > 16/17 March 1943 </td><td colspan=1 rowspan=1 > Zaanland </td><td colspan=1 rowspan=1 > Netherlands </td><td colspan=1 rowspan=1 > 0 </td><td colspan=1 rowspan=1 > 6,513 </td><td colspan=1 rowspan=1 > U-758 </td></tr>
<tr> <td colspan=1 rowspan=1 > 16/17 March 1943 </td><td colspan=1 rowspan=1 > Southern Princess </td><td colspan=1 rowspan=1 > United Kingdom </td><td colspan=1 rowspan=1 > 4 </td><td colspan=1 rowspan=1 > 12,156 </td><td colspan=1 rowspan=1 > U-600 </td></tr>
<tr> <td colspan=1 rowspan=1 > 16/17 March 1943 </td><td colspan=1 rowspan=1 > Harry Luckenbach </td><td colspan=1 rowspan=1 > United States </td><td colspan=1 rowspan=1 > 80 </td><td colspan=1 rowspan=1 > 6,366 </td><td colspan=1 rowspan=1 > U-91 </td></tr>
<tr> <td colspan=1 rowspan=1 > 16/17 March 1943 </td><td colspan=1 rowspan=1 > Coracero </td><td colspan=1 rowspan=1 > United Kingdom </td><td colspan=1 rowspan=1 > 5 </td><td colspan=1 rowspan=1 > 7,252 </td><td colspan=1 rowspan=1 > U-384 </td></tr>
<tr> <td colspan=1 rowspan=1 > 16/17 March 1943 </td><td colspan=1 rowspan=1 > Terkoeli </td><td colspan=1 rowspan=1 > Netherlands </td><td colspan=1 rowspan=1 > 36 </td><td colspan=1 rowspan=1 > 5,158 </td><td colspan=1 rowspan=1 > U-631, U-384? </td></tr>
<tr> <td colspan=1 rowspan=1 > 17 March 1943 </td><td colspan=1 rowspan=1 > James Oglethorpe </td><td colspan=1 rowspan=1 > United States </td><td colspan=1 rowspan=1 > 44 </td><td colspan=1 rowspan=1 > 7,176 </td><td colspan=1 rowspan=1 > U-758, U-91 </td></tr>
<tr> <td colspan=1 rowspan=1 > 17 March 1943 </td><td colspan=1 rowspan=1 > William Eustis </td><td colspan=1 rowspan=1 > United States </td><td colspan=1 rowspan=1 > 0 </td><td colspan=1 rowspan=1 > 7,196 </td><td colspan=1 rowspan=1 > U-435, U-91 </td></tr>
<tr> <td colspan=1 rowspan=1 > 17 March 1943 </td><td colspan=1 rowspan=1 > Nariva </td><td colspan=1 rowspan=1 > United Kingdom </td><td colspan=1 rowspan=1 > 0 </td><td colspan=1 rowspan=1 > 8,714 </td><td colspan=1 rowspan=1 > U-600, U-91 </td></tr>
<tr> <td colspan=1 rowspan=1 > 17 March 1943 </td><td colspan=1 rowspan=1 > Irenee du Pont </td><td colspan=1 rowspan=1 > United States </td><td colspan=1 rowspan=1 > 24 </td><td colspan=1 rowspan=1 > 6,125 </td><td colspan=1 rowspan=1 > U-600, U-91 </td></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > 18 March 1943 </td><td class="highlighted"  colspan=1 rowspan=1 > William Q Gresham </td><td colspan=1 rowspan=1 > United States </td><td colspan=1 rowspan=1 > 27 </td><td colspan=1 rowspan=1 > 7,191 </td><td class="highlighted"  colspan=1 rowspan=1 > U-221 </td></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > 18 March 1943 </td><td class="highlighted"  colspan=1 rowspan=1 > Canadian Star </td><td colspan=1 rowspan=1 > United Kingdom </td><td colspan=1 rowspan=1 > 29 </td><td colspan=1 rowspan=1 > 8,293 </td><td class="highlighted"  colspan=1 rowspan=1 > U-221 </td></tr>
<tr> <td colspan=1 rowspan=1 > 19 March 1943 </td><td colspan=1 rowspan=1 > Matthew Luckenbach </td><td colspan=1 rowspan=1 > United States </td><td colspan=1 rowspan=1 > ? </td><td colspan=1 rowspan=1 > 5,848 </td><td colspan=1 rowspan=1 > U-523, U-527 </td></tr>
</table>
Question:
Which HX 229 ships were sunk by U-221 and when?
Answer:
On 18 March, U-221 sunk two ships of HX 229 (William Q Gresham and Canadian Star).