Approximately, from just a student: When things are singular you use "du" and "dig". When it gets plural you use "ni" and "er". They all mean "you", du and ni are for the subject of a sentence, dig and er are for the object.
It´s "for yourselves" because ni is the plural you. But I´m not sure whether or not that would be accepted anyway. Seems to me "ha [time] på sig" is a fixed expression and your version adds new meaning to the phrase.