How to know the release date of my BIOS version?

Easily use the command dmidecode.

First install if you haven't do yet:

sudo apt-get install dmidecode

Now to check the release date of BIOS easily run the command:

sudo dmidecode --string bios-release-date

The output will look like(my output):

07/23/2012

dmidecode is indeed the easiest way but it should be used with caution. As mentioned at the end of man dmidecode:

BUGS
   More often than not, information contained in the DMI tables is inaccu‐
   rate, incomplete or simply wrong.

So, the authors themselves warn the user that the information returned might be crap.

Another program that is useful for getting information about the BIOS (though not the release date) is biosdecode which used to be part of dmidecode but has now been separated from it. Example output:

# biosdecode 2.12
SMBIOS 2.6 present.
    Structure Table Length: 3650 bytes
    Structure Table Address: 0x000F2430
    Number Of Structures: 67
    Maximum Structure Size: 253 bytes
PCI Interrupt Routing 1.0 present.
    Router ID: 00:1f.0
    Exclusive IRQs: None
    Compatible Router: 8086:2912
    Slot Entry 1: ID 00:01, on-board
    Slot Entry 2: ID 00:02, on-board
    Slot Entry 3: ID 00:1f, on-board
    Slot Entry 4: ID 00:1d, on-board
    Slot Entry 5: ID 00:1a, on-board
    Slot Entry 6: ID 00:1b, on-board
    Slot Entry 7: ID 00:1c, on-board
    Slot Entry 8: ID 00:19, on-board
    Slot Entry 9: ID 02:00, slot number 33
    Slot Entry 10: ID 03:00, slot number 34
    Slot Entry 11: ID 04:00, slot number 8
    Slot Entry 12: ID 06:00, slot number 9
    Slot Entry 13: ID fe:00, slot number 10
    Slot Entry 14: ID fe:00, slot number 16
    Slot Entry 15: ID fe:00, slot number 17
    Slot Entry 16: ID 00:16, on-board
    Slot Entry 17: ID 0c:03, slot number 4
    Slot Entry 18: ID 0c:02, slot number 3
    Slot Entry 19: ID 0c:00, slot number 1
    Slot Entry 20: ID 0c:01, slot number 2
    Slot Entry 21: ID 0c:05, slot number 2
    Slot Entry 22: ID 00:03, on-board
    Slot Entry 23: ID 00:04, on-board
    Slot Entry 24: ID 00:05, on-board
    Slot Entry 25: ID 00:06, on-board
    Slot Entry 26: ID fe:00, slot number 1
    Slot Entry 27: ID fe:00, slot number 3
    Slot Entry 28: ID 00:00, on-board
PNP BIOS 1.0 present.
    Event Notification: Polling
    Event Notification Flag Address: 0x000004B4
    Real Mode 16-bit Code Address: F000:E2F1
    Real Mode 16-bit Data Address: 0040:0000
    16-bit Protected Mode Code Address: 0x0001D2F6
    16-bit Protected Mode Data Address: 0x00000400
ACPI 2.0 present.
    OEM Identifier: DELL  
    RSD Table 32-bit Address: 0xCF67DF18
    XSD Table 64-bit Address: 0x00000000CF67DE18
BIOS32 Service Directory present.
    Revision: 0
    Calling Interface Address: 0x000FFA10