The difference between 'jail time' and 'prison time', at least in the way I use it, is that jail in the US is for short sentences or as a holding cell for an upcoming trial, while prison is reserved for after sentencing. In other words, when someone is sentenced to '30 days in jail', that time is normally spent in the local facility; a prison sentence usually is in terms of years rather than days and that prison can be anywhere in the country.