Mutual promises, concurrent considerations, which will support each other, unless one or the other be void; in which case, there being no consideration on the one side, no contract can arise. But if the promise on one side be only voidable, as in consideration of money given or of a promise by an infant, it is sufficient.
Mutual promises, however, to be obligatory, must be made simultaneously. If they be made at different times on the same day they will not be a good consideration for each other because of the want of reciprocity of obligation at the moment the contract is made, Story on Contracts