Seconds in a day. Should be 86400.

Regardless, mhs as a unit doesn't make too much sense. kwh (3600kJ) came to be because it can allow for easy mental math (e.g. 100w light bulb x 24h operation = 2400wh = 2.4kwh). Otherwise, if your electric rate were simply $ per kJ, you'd have to do more calculations.

For our case of profitability, the current standard of btc/day per mh/s makes perfect sense, other than being long to express. You simply take your hashrate in mh/s and multiply it by this profitability and you get your expected btc/day.