not sure why it would return value, unless the value in D3 is actually
text in which case you'll have to convert it to a date.

if you want to know how long, to the second, is left, you will have to
create a string yourself, such as:

=DAY(D3-NOW()) & " days, " & HOUR(D3-NOW()) & " hours, " &
MINUTE(D3-NOW()) & " minutes, " & SECOND(D3-NOW()) & " seconds"

The pattern should be obvious so you can make it as complex as you
need.

Rob