Instead of flagging, I would suggest you use conditional formatting. In this example, any date that's prior to today's date is highlighted in red; anything ending within 30 days is yellow. It's a much clearer visual cue. Today's date is in D2, for reference.
<b>sheet</b><br /><br /><table border="1" cellspacing="0" cellpadding="0" style="font-family:Calibri,Arial; font-size:11pt; background-color:#ffffff; padding-left:2pt; padding-right:2pt; "> <colgroup><col style="font-weight:bold; width:30px; " /><col style="width:71px;" /><col style="width:64px;" /><col style="width:64px;" /><col style="width:64px;" /></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></tr><tr style="height:36px ;" ><td style="font-size:8pt; background-color:#cacaca; text-align:center; " >1</td><td style="font-family:Arial; font-size:12pt; text-align:left; ">Start Date</td><td style="font-family:Arial; font-size:12pt; text-align:left; ">End Date</td><td style="font-family:Arial; font-size:10pt; ">*</td><td style="font-family:Arial; font-size:10pt; ">*</td></tr><tr style="height:15px ;" ><td style="font-size:8pt; background-color:#cacaca; text-align:center; " >2</td><td style="font-family:Arial; font-size:10pt; text-align:left; ">2/24/2010</td><td style="font-family:Arial; font-size:10pt; text-align:left; ">6/1/2010</td><td style="font-family:Arial; font-size:10pt; ">*</td><td style="font-family:Arial; font-size:10pt; text-align:right; ">3/3/2010</td></tr><tr style="height:15px ;" ><td style="font-size:8pt; background-color:#cacaca; text-align:center; " >3</td><td style="font-family:Arial; font-size:10pt; text-align:left; ">8/18/2008</td><td style="background-color:#ff0000; font-family:Arial; font-size:10pt; text-align:left; ">6/24/2009</td><td style="font-family:Arial; font-size:10pt; ">*</td><td style="font-family:Arial; font-size:10pt; ">*</td></tr><tr style="height:15px ;" ><td style="font-size:8pt; background-color:#cacaca; text-align:center; " >4</td><td style="font-family:Arial; font-size:10pt; text-align:left; ">12/21/2009</td><td style="background-color:#ffff00; font-family:Arial; font-size:10pt; text-align:left; ">3/15/2010</td><td style="font-family:Arial; font-size:10pt; ">*</td><td style="font-family:Arial; font-size:10pt; ">*</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>Spreadsheet Formulas</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 >=TODAY()</td></tr></table></td></tr></table><br /><table style="font-family:Arial; font-size:10pt; background-color:#fffcf9; border-style: groove ;border-color:#ff0000"><tr><td ><b>Conditional formatting </b></td></tr><tr><td ><table border="1" cellspacing="0" cellpadding="0" style="font-family:Arial; font-size:10pt; padding-left:2pt; padding-right:2pt; "><tr><td >Cell</td><td >Nr.: / Condition</td><td >Format</td></tr><tr><td >B2</td><td >1. / Formula is =B2<TODAY()</td><td style="background-color:#ff0000; ">Abc</td></tr><tr><td >B2</td><td >2. / Formula is =B2<(TODAY()+30)</td><td style="background-color:#ffff00; ">Abc</td></tr></table></td></tr></table>
You can use the same principle to flag in a separate column, if you want.