A one day declaration game combines the traditional innings format with the limited overs format. It’s sometimes played in school games to help develop the tactics of first class cricket despite constraints on playing time.
A “declaration game” is simply one scheduled for a particular number of days and innings, with an innings only ending when the team is all out, or the captain declares. There’s no time limit on individual innings. (First Class, List A and Test matches are all forms of declaration game.)
Such games usually take more than one day, making them difficult to play in games between schools or amateur clubs. One solution is a one day declaration game. This means the total length of the game is fixed (eg 110 overs), but not the individual innings. In theory the first team batting could do so for as long as they liked before being all out. In practice, the team will usually decide when to declare, gambling that it has enough runs to win the game but enough overs left to bowl out the other team. This is effectively the same judgement that is needed in first class cricket.
One variation is to say the first team batting must declare by the halfway point of the scheduled overs (eg 55 overs in a 110 over game). The twist is that the game still has the same total time allocation, regardless of when the first team declares. For example, if it declared after 40 overs in a 110 over game, the opponents would have a maximum of 70 overs for their innings.
This variation works well with teams of differing skill levels, particularly if the better team bats first. If it declares too early, it risks giving the opponents more time to chase a lower target. If it waits until the last possible moment to declare, it risks not having enough time to bowl out the opponents.