As other reviewers have noted, this app lets you convert between gregorian and japanese era calendars. It does this by presenting two spinning wheels (one with the western year, the other with the japanese year) which are locked to one another - you can spin either one and the other will turn to match, letting you look up the corresponding date. Of course, you *could* do this in your head, especially for the most current era (heisei) since you always know what the most recent year of the era is, but once you start getting into past eras, it becomes increasingly difficult unless you have the exact years of the emperor's death and the length of their reign memorised, something which most Japanese, especially young people, do not. For this reason, most Japanese day planners come with a small conversion chart in the back for reference. If you live in Japan and have such a chart you probably won't need this application but if not, and you find yourself needing to convert between the two calendar systems, it can be quite handy. At any rate, it's free, so you can't go wrong!