Please use ***
[code][/code]*** tags around your code and data, to preserve the original formatting and to improve readability. Do
not use quote tags, bolding, colors, "start/end" lines, or other creative techniques.
Yes,
bash is integer only, as is
expr, which is what you're trying to use.
(
expr is completely unneeded in modern shells, btw. Everything it can do is available built in. I suggest just forgetting that it exists.

)
For floating point arithmetic you need to use an external command like
bc or
awk.
arithmetic expressions
How can I calculate with floating point numbers instead of just integers?
You also have two completely unnecessary uses of
echo (the commands for which were unclosed as well),
and
$(..) is highly recommended over `..`.
Code:
var1=timestamp:n:1111111.2222222
var2=timestamp:n:3333333.4444444
start_time=${var1#*:n:}
stop_time=${var2#*:n:}
render_time=$( echo "$stop_time - $start_time" | bc )
echo "$stop_time - $start_time = $render_time"