"Time Since/Ago" Library for Android/Java

Check out DateUtils.getRelativeTimeSpanString(), it's been around since API level 3.

From the Google I/O 2012 App:

/*
 * Copyright 2012 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

private static final int SECOND_MILLIS = 1000;
private static final int MINUTE_MILLIS = 60 * SECOND_MILLIS;
private static final int HOUR_MILLIS = 60 * MINUTE_MILLIS;
private static final int DAY_MILLIS = 24 * HOUR_MILLIS;


public static String getTimeAgo(long time, Context ctx) {
    if (time < 1000000000000L) {
        // if timestamp given in seconds, convert to millis
        time *= 1000;
    }

    long now = getCurrentTime(ctx);
    if (time > now || time <= 0) {
        return null;
    }

    // TODO: localize
    final long diff = now - time;
    if (diff < MINUTE_MILLIS) {
        return "just now";
    } else if (diff < 2 * MINUTE_MILLIS) {
        return "a minute ago";
    } else if (diff < 50 * MINUTE_MILLIS) {
        return diff / MINUTE_MILLIS + " minutes ago";
    } else if (diff < 90 * MINUTE_MILLIS) {
        return "an hour ago";
    } else if (diff < 24 * HOUR_MILLIS) {
        return diff / HOUR_MILLIS + " hours ago";
    } else if (diff < 48 * HOUR_MILLIS) {
        return "yesterday";
    } else {
        return diff / DAY_MILLIS + " days ago";
    }
}