Group by 15 Minute Steps

Hi,

does anyone hava an idea how to group data e.g by 15 minute steps?
I have the following data in my "data_diff" table

[...]
snmp=# \d data_diff                              Table "public.data_diff"    Column      |  Type   |
    Modifiers
 
-----------------+---------+-----------------------------------------------------------id              | integer | not
nulldefault
 
nextval('public.data_diff_id_seq'::text)device          | integer |psqlzeit        | integer |snmpzeit        | integer
|ifinoctets     | bigint  |ifoutoctets     | bigint  |
 

Indexes:   "data_diff_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:   "data_diff_device_fkey" FOREIGN KEY (device) REFERENCES device(id)
ON UPDATE CASCADE ON DELETE CASCADE
[...]

An example query looks like this:

snmp=# select snmpzeit as snmp,ifinoctets,ifoutoctets from data_diff
where device=5 order by psqlzeit desc;
   snmp    | ifinoctets | ifoutoctets
------------+------------+-------------1111268704 |     111382 |      2805661111268405 |      78874 |
225921111268104|      88645 |       328031111267804 |      76273 |       19024
 

...
1111255204 |     149963 |       628891111254904 |     164788 |       815941111254604 |     147720 |
620931111254305|     161958 |       788131111254005 |     155495 |       68143
 


Usally, the steps between each entries is 300 seconds.
How can I group by by 15, 30, 45 minutes so that i can get averages over
the specified timeframe?

Greetings,
Martin