It's not so strange as "ETA" refers to a column, and is not a string actually. Anyway, odd that this happens. A quick fix may be to edit the file functions/functions.php (under /usr/share/urd/ if using the .deb file) and at line 445 in function store_ETA() you see a line like
$eta = floor($eta);
After that line add a line like:
$eta = max(0, $eta);
Thanx for reporting this tho.