DateTime: Difference between Hour and Integer

I have some mistakes, in my code in the 2 lines where a comment above them:

How can I change my code to find the best result ?

Code should be similar to my answer to you here, but you need to understand what I did. You definitely need to check api calls I used, but I added some additional comments:

import java.time.temporal.ChronoUnit
import java.time.LocalTime

import scala.concurrent.duration._

val t = LocalTime.now()


// start of the day
val start = LocalTime.of(9, 0)
// end of first half
val midEnd = LocalTime.of(13, 0)
// start of second half
val midStart = LocalTime.of(14, 0)
// end of the day
val end = LocalTime.of(18, 0)
// here we define duration of first half a day: diff between start of a day and midEnd (end of first half)
val firstHalf = start.until(midEnd, ChronoUnit.MILLIS).millis
// here we define duration of second half a day: diff between start of second half a day and end of a day
val secondHalf = midStart.until(end, ChronoUnit.MILLIS).millis

def toStart(t: LocalTime) = {
  // when checked time is before start of a day
  if (t.isBefore(start)) 0.hours
  // otherwise when checked time is before end of first half (will be diff between start time and checked time)
  else if (t.isBefore(midEnd)) start.until(t, ChronoUnit.MILLIS).millis
  // otherwise when checked time is before start of second half (will be duration of first half)
  else if (t.isBefore(midStart)) firstHalf
  // otherwise when checked time is before end of a day (will be duration of first half + duration of diff between checked time and start of second half)
  else if (t.isBefore(end)) firstHalf + midStart.until(t, ChronoUnit.MILLIS).millis
  // otherwise sum of durations 
  else firstHalf + secondHalf
}

// here you can add any specific format for evaluated duration
implicit class formatter(d: FiniteDuration) {
  def withMinutes = {
    // convert to minutes
    val l = d.toMinutes
    // format
    s"${l / 60}:${l % 60}"
  }
}

toStart(t).withMinutes
toStart(LocalTime.of(9, 30)).withMinutes
toStart(LocalTime.of(12, 30)).withMinutes
toStart(LocalTime.of(13, 30)).withMinutes
toStart(LocalTime.of(14, 30)).withMinutes

Spend some time and check java.time api (specifically LocalTime.until). Check FiniteDuration api to understand .millis suffix I used