Creating massive lines of repeaters or complicated minecart through web is fussy and takes up too much space if you're trying to make a timer system for an adventure map, and they can't always be precise and resistant to updates. Which is why I propose a /wait command.
What does it do?
The /wait command would be limited to command blocks and would work like so,
/wait [time in redstone ticks]
It's that simple! So, once you power a command block with this command in it, it will wait the specified time and then output a regular pulse, like from a button, out of any comparators attached to it. Easy. If you send a second signal into the command block while it is waiting to output another single, it will just keep track of both.
How could it work?
I propose that it could simple use NBT data tags that is by default set at -1 so it doesn't output constantly, and when you use the command it sets the tag to the entered number and once powered it goes down by one every second. If a tag is counting down it is locked. Once it reaches 0 it outputs a signal and resets so that if a second pulse is added the tag can be reused. If a second pulse is added and the original tag is locked, the game will make a new tag and delete if after use to save space. This system means that it cannot lose track if you log out. There are already in-game NBT Editors so it is proven possible to update them live.
The fact that it could be updated a tag every second on logs of tags means that it could severely lag worlds, but this is also true of long lines of repeaters causing lighting updates. This could be countered if needed by setting a time/amount limit in your world file. This would not be a griefing issue on creative servers as you need to be opped to obtain and use command blocks.
NBT is only written when chunks are unloaded, the server is shutting down, or (in SSP) when the player is in a game-pausing GUI such as the options menu. Ticking such a timer would presumably write to a normal variable in the tile entity instance, which takes a millisecond or two. So, lag isn't an issue.
I know this is an old thread, but at the moment, I'm really needing this right now. I wish Mojang would add this and make my life as a map maker a whole lot easier. Let's revive this thread and have this in Minecraft 1.9 Mojang!