Well, now I know what happens. The day didn't increment properly. It works before dawn if I revert all of the $UVptr-1 entries to $UVptr, comment out the initial $UVptr++ in the PM portion of the conditional, and uncomment the third day's $UVptr++ like so:
$UVptr = 0; // index for which day to use
?>
<tr>
<td class="datahead" style="text-align:center"><?php echo $leftHead; ?></td>
<td class="datahead" style="text-align:center"><?php echo $rightHead; ?></td>
</tr>
<tr>
<td align="center">
<table width="180" border="0" cellpadding="2" cellspacing="0">
<?php
if (($haveUV) && ($dayornight == 'Day')) { // Have a UV sensor .. show realtime data & not night
?>
<tr>
<td align="center" width="35%" nowrap="nowrap" class="data2" style="text-align: center; font-size: 9px;">
<span class="ajax" id="ajaxuv" style="font-size: 14px;"><?php echo $VPuv; ?></span>
<div style="padding-top: 3px;"><?php langtrans('Sunburn in'); ?></div>
<span class="ajax" id="ajaxuvburnrate"><?php echo $burntime; ?></span> <?php langtrans('Minutes'); ?>
</td>
<td width="30%" align="center">
<span class="ajax" id="ajaxuvimg">
<img src="/ajax-images/uv<?php echo round($VPuv,0); ?>.gif" height="65" width="34" alt="" title="Current UV rate" />
</span>
</td>
<td width="35%" class="data2" style="text-align:center; font-size: 9px;">
<?php langtrans('Highest'); ?>:<br />
<?php echo $highuvtime; ?><br />
<span style="font-size: 14px;"><?php echo $highuv; ?></span>
</td>
</tr>
<?php } else { // don't have UV sensor .. show UV forecast instead ?>
<?php
if ($haveUV) { // Show todays high if evening or yesterdays if morning
?>
<tr>
<td align="center" colspan="3" nowrap="nowrap" class="data2" style="text-align: center; font-size: 10px;">
<?php if ($time_hour > 12) { // Display Summary if in the evening ?>
<?php langtrans('High Today'); ?>: <span style="font-size: 9px;">
<?php echo $highuv; ?> @ <?php echo $highuvtime; ?></span>
<?php $UVptr++; // increment counter ?>
<?php } else { ?>
<?php langtrans('High Yest'); ?>: <span style="font-size: 9px;">
<?php echo $highuvyest; ?> @ <?php echo $highuvyesttime; ?></span>
<?php } ?>
</td>
</tr>
<?php } ?>
<tr>
<td align="center" width="35%" nowrap="nowrap" class="data2" style="text-align: center; font-size: 9px;">
<?php $UVshortdate = mktime(0,0,0,date("m"), date(d)+$UVptr, date("y")); ?>
<?php echo date("M-d",$UVshortdate) ?><br/>
<img src="/ajax-images/uv<?php echo round($UVfcstUVI[$UVptr],0); ?>.gif" height="45" width="34" alt="" title="Predicted UV rate" /><br/>
<b><a href="<?php echo htmlspecialchars($UV_URL); ?>" title="<?php echo strip_tags($requiredNote); ?>"><?php echo $UVfcstUVI[$UVptr]; ?></a></b>
<?php $UVptr++; // increment counter ?>
</td>
<td align="center" width="30%" class="data2" nowrap="nowrap" style="text-align: center; font-size: 9px;">
<?php $UVshortdate = mktime(0,0,0,date("m"), date(d)+$UVptr, date("y")); ?>
<?php echo date("M-d",$UVshortdate) ?><br/>
<img src="/ajax-images/uv<?php echo round($UVfcstUVI[$UVptr],0); ?>.gif" height="45" width="34" alt="" title="Predicted UV rate" /><br/>
<b><a href="<?php echo htmlspecialchars($UV_URL); ?>" title="<?php echo strip_tags($requiredNote); ?>"><?php echo $UVfcstUVI[$UVptr]; ?></a></b>
<?php $UVptr++; // increment counter ?>
</td>
<td align="center" width="35%" class="data2" nowrap="nowrap" style="text-align: center; font-size: 9px;">
<?php $UVshortdate = mktime(0,0,0,date("m"), date(d)+$UVptr, date("y")); ?>
<?php echo date("M-d",$UVshortdate) ?><br/>
<img src="/ajax-images/uv<?php echo round($UVfcstUVI[$UVptr],0); ?>.gif" height="45" width="34" alt="" title="Predicted UV rate" /><br/>
<b><a href="<?php echo htmlspecialchars($UV_URL); ?>" title="<?php echo strip_tags($requiredNote); ?>"><?php echo $UVfcstUVI[$UVptr]; ?></a></b>
<?php $UVptr++; // increment counter ?>
</td>
</tr>
<?php //$UVptr++; // increment counter ?>
My addled brain failed to notice that I had effectively neutralized the first $UVptr++ statement with my $UVptr-1 modifications. I also noticed that WWL-mobile.php was skipping tomorrow's date before midnight last night but is correct early this AM, so I also commented out the $UVptr++ in the PM portion of WWL-mobile.php. Dawn and PM will bring new challenges, I'm sure.
Edit May29 10PM CDT
1) Forgot to watch the clock as I worked past midnight. Removed all $UVptr++ modifications in ajax-dashboard6.php and WWL-mobile.php this evening except the last one in ajax-dashboard6.php in line 1463 because I was unaware of the actual date! All of my icons are now displaying with the correct date and data.
2) The icon directory modifications in ajax-dashboard6.php remain in place and it's working correctly, I think.
3) I changed the above code to what is working for me.