It appears as a bundle of dynamite, wired to a clock. When armed and thrown, it will wait for 10 seconds, after which it will detonate. Another option is to hold the alternate fire button down. This will cause a digital display on the ammunitions display to count up. When the desired number of seconds is achieved, the alternate fire button is released and the bomb is thrown.
The Time Bomb is best placed in frequent traffic places, or when the player knows an enemy will pass shortly. It can also be thrown in or in front of (enemy) vehicles like jeeps or tanks.
Because Battlefield Vietnam features different loadouts for each faction, this list may not be specific enough. Please see each faction's respective page for information on individual weapon kits and loadouts.