var TimeReg = /^(0?[1-9]|1[012]):([0-5]d) ?([APap][mM])$/

function timeDiff(stimeIn, stimeOut) {
    var timein = TimeReg.exec(stimeIn)
    var timeout = TimeReg.exec(stimeOut)
    var isPM, hrs, mins
    if (timein != null && timeout != null) {
        isPM = timein[3].toLowerCase() == 'pm'
        hrs = (isPM) ? 12 : 0
        hrs = (timein[1] == '12' && isPM) ? 0 : hrs
        mins = (timein[1] == '12' && !isPM) ? 0 : 60
        timein = (timein[1] * 1 + hrs) * mins + timein[2] * 1

        isPM = timeout[3].toLowerCase() == 'pm'
        hrs = (isPM) ? 12 : 0
        hrs = (timeout[1] == '12' && isPM) ? 0 : hrs
        mins = (timeout[1] == '12' && !isPM) ? 0 : 60
        timeout = (timeout[1]*1 + hrs) * mins + timeout[2] * 1
        return (timeout - timein >= 0) ? (timeout - timein)  : -1
    } else {
        return -1
    }
}

timeDiff("9:30am","4:00pm")  // 390
timeDiff("12:00am","1:00pm") //780
timeDiff("9:30pm","4:00pm")  // -1  not valid
timeDiff("9:30an","4:00pm")  // -1  not valid