Construction of Nonce

Hello,

I would have a question about Nonce construction.
What is the meaning of "+" in computation formula (nonce = time + serial 
+ M ( time + serial , k ))?
It looks like concatenation. But when looking at examples 
("Rj2BoUZM7PisPX2ytSAAWA==" in response 1), beginning of Nonce does not 
seem to look like "clear" time value. Moreover, I think concatenation of 
"clear" values and encrypted values would not be efficient to avoid 
leakink information [74].
So perhaps formula is nonce = MAC of time&serial concatenation with k 
key (nonce = M ( time + serial , k ) rather, in my mind). But in this 
case, how time or serial values could be retrieved from MAC encrypted 
value ?

I understand that this formula is not mandatory for service 
implementation, but I would like to use it to be nearer from specification.

Sincerely,

Frédéric Deléon