Relative time utils

RelativeTimeUtils.kt

/**
 * Parse a local date time string into a UTC date time object
 * TODO: update to Java 8 time API https://docs.oracle.com/javase/8/docs/api/java/time/package-summary.html, return java.time.Instant
 */
fun String.toDate(
    dateFormat: String = "yyyy-MM-dd'T'HH:mm:ss",
    timeZone: TimeZone = TimeZone.getTimeZone("UTC")
): Date? {
    val parser = SimpleDateFormat(dateFormat, Locale.getDefault())
    parser.timeZone = timeZone
    return parser.parse(this)
}

fun String.toRelativeTimeStr(
    now: Long = System.currentTimeMillis(),
    format: Int = FORMAT_ABBREV_ALL
): CharSequence? {
    return toDate()?.time?.toRelativeTimeStr(now, format)
}

fun Long.toRelativeTimeStr(
    now: Long = System.currentTimeMillis(),
    format: Int = FORMAT_ABBREV_ALL
): CharSequence? {
    val timeDelta = now - this
    return  when {
        timeDelta < MINUTE_IN_MILLIS -> "Just Now"
        timeDelta < HOUR_IN_MILLIS -> getRelativeTimeSpanString(
            this,
            now,
            MINUTE_IN_MILLIS,
            FORMAT_ABBREV_ALL
        )
        timeDelta < DAY_IN_MILLIS -> getRelativeTimeSpanString(
            this,
            now,
            HOUR_IN_MILLIS,
            FORMAT_ABBREV_ALL
        )
        else -> getRelativeTimeSpanString(
            this,
            now,
            DAY_IN_MILLIS,
            format
        )
    }
}

RelativeTimeUtilsTest.kt

@RunWith(AndroidJUnit4::class)
class StringExtensionsTest {

    // Aug 11 2021 19:28:53 UTC
    private val now = 1628710133395

    @Test
    fun `given a date string in the past month, return relative time string to now`() {
        val dateStr = "2021-07-26T20:17:00.115Z"

        val result = dateStr.toRelativeTimeStr(now)

        assertEquals("Jul 26", result?.toString())
    }

    @Test
    fun `given a date string in the past 24+ hours, return relative time string to now`() {
        val dateStr = "2021-08-10T09:09:00.115Z"

        val result = dateStr.toRelativeTimeStr(now)

        assertEquals("Yesterday", result?.toString())
    }

    @Test
    fun `given a date string in the past 3 days, return relative time string to now`() {
        val dateStr = "2021-08-08T20:17:00.115Z"

        val result = dateStr.toRelativeTimeStr(now)

        assertEquals("3 days ago", result?.toString())
    }

    @Test
    fun `given a date string in the past 3 hours, return relative time string to now`() {
        val dateStr = "2021-08-11T16:00:00.115Z"

        val result = dateStr.toRelativeTimeStr(now)

        assertEquals("3 hr. ago", result?.toString())
    }

    @Test
    fun `given a date string in the past hour, return relative time string to now`() {
        val dateStr = "2021-08-11T19:25:00.000Z"

        val result = dateStr.toRelativeTimeStr(now)

        assertEquals("3 min. ago", result?.toString())
    }

    @Test
    fun `given a date string within a minute, return relative time string to now`() {
        val dateStr = "2021-08-11T19:28:00.000Z"

        val result = dateStr.toRelativeTimeStr(now)

        assertEquals("Just Now", result?.toString())
    }