I'm a game reviewer for a website and when we review games, we like to include the number of hours invested into the game to warrant our opinions posted. I am currently reviewing Kirby Mass Attack for the DS and Kirby's Return to Dreamland for the Wii. The Wii has an excellent system of keeping track of hours played for any given title. It even includes separate times for multiple titles played in a given day. Why doesn't the portable systems like the DSi and 3DS have any methods of monitoring the time spent playing games?? One would think there would be something in line with this under Parental Controls so parents can keep track of how long their kids spend gaming. Is there any way that this could be something that could be added via a system update in the future? I just find it odd that Nintendo would have this feature on the Wii, but not on portable systems as well.
The 3DS does in fact have this functionality.
One of the stock tiles on the homescreen is the "Activity Log," which will show a history of games played, including how long you have played them.
The Nintendo 3DS as far as I know does moniter the time spent playing video games in the Activity Log application on the Home Menu. You can see all that you have played since you bought the 3DS system and the ranking for how long you played games over a day, week, month, or year. You can see all of the games you played and sort them by rankings such as play time, play frequency, average time played, and date. The Activity Log has so many functions that I can't even list them all here. These are only some of the ways that the 3DS moniters how you play. It's a lot more advanced than the Wii. I'm surprised that you missed this.
NOTE: This only applies to the Nintendo 3DS.
I don't have a Nintendo DSi or DSi XL.
In addition to what the others have stated, it should be pointed out that the Activity Log on the Nintendo 3DS will also keep track of Nintendo DS, Nintendo DSi, and Nintendo DSiWare game records as well. It's not just for Nintendo 3DS games and apps.