Understanding time format of the EXPLAIN command - Postgres

Question

When I run the EXPLAIN ANALYZE command on a given query, I'm having a difficult time interpreting the outputted time value. For example (actual time=8163.890..8163.893). Do the internal decimals represent repeating characters?? Sorry, this may be a noobish question, but I want to make sure I'm interpreting the results correctly.

   ->  GroupAggregate  (cost=2928781.21..2929243.02 rows=1 width=27712) (actual time=8163.890..8163.893 rows=1 loops=1)

Answer 1

actual time=8163.890..8163.893 means

Initializing that step ("startup") took 8163.890ms
Running the whole step took 8163.893ms

So in that case nearly the complete work was done in the startup phase of that step.

Edit:
The same logic is "applied" to the cost information

cost=2928781.21..2929243.02 means:

The cost to initialize this step was estimated at: 2928781.21
The cost to perform the step was estimated at: 2929243.02

(note that "cost" does not have a unit - it's an arbitrary value)

This is also explained here: http://www.postgresql.org/docs/current/static/using-explain.html

Answer 2

The first number is how long it took to return the first row in that step. The second number is how long it took to return the last row.