The tax year in Taiwan runs from January to December. For the first six months (183 days) that you are in Taiwan, you will need to pay 18% tax on your earnings. This period of time is determined by the entry/exit stamps in your passport, and not your employment contract. You then file for tax returns before May 31st of the following year. However, if you arrive after July 31st, you will not be in this bracket. You will therefore still need to pay the 18% tax, but will not be able to file for tax returns on it. If you arrive before the July 31st cutoff date and are present in Taiwan for longer than 183 days in that tax year (for example, if your arrival date was in March), then you can claim a partial refund on the taxes paid in the following year. Be careful as any time spent out of the country (visa runs, holidays, etc.) will not count toward this 183-day period.
Starting 2012, certified teachers are no longer exempt from income tax, so their income is also subject to the same tax rate.
For those who are leaving before their tax refunds arrive: You can assign a local as your proxy to accept this on your behalf. Please check with your local tax office for the form you need to complete to do so.