If that date of 28 August is your enter before date, you will need to purchase a reentry permit. Unless you plan to make other trips out of the country a single reentry permit will take care of your return after your departure.
Or one could apply for a non imm o-a multi entry visa which would give the user a year, and with a border run just prior to the visa enter before date, the user could get a second year before needing to deal with extension of stay applications.
Krishn Menaria the non imm o-a multi entry visa is obtained via a Thai Embassy or official Thai Consulate in one’s home country for those 50 years and older. It is a one year visa with no need for border runs. However, each time the holder leaves Thailand prior to the enter before date stamped on the visa, when the holder re-enters Thailand he/she will be given an additional year from that date. So if the holder times thing correctly he/she could essentially get almost two years before needing to apply for extensions of stay at their local Thai immigration office. There are requirements to getting the visa - income/savings that can remain in one’s home country, a police check and a medical form that must be completed. For specific requirements I suggest you view the website of the embassy/consulate where you would be obtaining this visa - there are slightly different requirements depending on the specific embassy/consulate. I like it in that it provides some breathing room for those moving to Thailand before applying for extensions of stay etc. It is just another possible option.