Can i take alka seltzer day and night if it expired last month ? i don't have money to buy a new box ! but is it safe ?

No issue: most medications are ok five years after expiration. Especially Alka Seltzer. Obviously, drug companies do not want us to know. Do not worry. Exception is the epipen (epinephrine).

Yes: Alka-Seltzer is a combination of sodium bicarbonate, aspirin, and anhydrous citric acid, used for the relief of heartburn, acid indigestion, and stomach aches. In general most medicines are still effective a few months after expiration dates. However, acids do tend to destabilize over time, so I would not use a 2014 box of asa or alkaseltzer in 2015, but it should be fine 1 month post expiration.