Gives an example of how I take a date that is GMT and convert it to the TimeZone that I want.

DateTesting.kt

/**
 * Given a supplied [dateString] that matches our [RFC_3339_FORMAT], convert it to a 
 * [Calendar] instance for the corresponding [timeZoneString]. 
 * 
 * The only way I've been able to do this is:
 * 1. Take the date string, which is GMT, and convert it to a Date.
 * 2. Update the TZ on our SimpleDateFormat, to get the string in the desired TimeZone.
 * 3. Reverse engineer that back into a date. 
 * 4. Return this calendar. 
 * 
 * The step to a string seems intermediary. Why can't I create a calendar at GMT timezone, and then
 * tell it to convert? 
 */
fun getCalendarWithTimeZone(
    dateString: String,
    timeZoneString: String
): Calendar {
    val dateFormatter = SimpleDateFormat(RFC_3339_FORMAT, Locale.getDefault())
    dateFormatter.timeZone = TimeZone.getTimeZone("GMT")
    val dateInGmt = dateFormatter.parse(dateString)

    val timeZone = TimeZone.getTimeZone(timeZoneString)
    dateFormatter.timeZone = timeZone
    val updatedDateString = dateFormatter.format(dateInGmt)
    val updatedDate = dateFormatter.parse(updatedDateString)

    val calendar = Calendar.getInstance(timeZone)
    calendar.time = updatedDate

    return calendar
}

class DateTesting {
    @Test
    fun convertTimeToNY() {
        val dateString = "2020-09-24T22:00:00Z"
        val timeZoneId = "America/New_York"

        val calendar = getCalendarWithTimeZone(dateString, timeZoneId)
        assertThat(calendar.get(Calendar.HOUR)).isEqualTo(6)
    }
}

Here is the same code using the java.time package. Note that I compared hour to 18 instead of 6, because I'm not sure how to get PM but that can be resolved later.

At least this is a cleaner way of converting dates:

fun java8Date(
    dateString: String,
    timeZoneString: String
): LocalDateTime {
    val gmtZone = ZoneId.of("GMT")
    val userZone = ZoneId.of(timeZoneString)

    val dateTimeFormatter = DateTimeFormatter.ofPattern(RFC_3339_FORMAT)
    val localDateInGMT = LocalDateTime.parse(dateString, dateTimeFormatter)
    val zonedDateGMT = ZonedDateTime.of(localDateInGMT, gmtZone)

    val userZonedDate = zonedDateGMT.withZoneSameInstant(userZone)

    return userZonedDate.toLocalDateTime()
}


class DateTesting {
    @Test
    fun convertTimeToNY() {
        val dateString = "2020-09-24T22:00:00Z"
        val timeZoneId = "America/New_York"

        val localDate = java8Date(dateString, timeZoneId)
        assertThat(localDate.hour).isEqualTo(18)
    }
}