Right. It means you enter on Jan 17, and you'll get sixty days (which can be extended another thirty in the country) from that date. If you enter Jan 19, that sticker is no longer valid, so you'll only be able to enter with a visa exemption stamp (assuming you're from a visa exemption country).
With a UK passport, you can do a border run (leave and come back) and get a thirty day Visa Exempt stamp. But you can extend that at an immigration office within the Kingdom for an additional thirty days. I know nothing about teenager visas.
This is Thailand? That's all I know. They are also scrutinizing paperwork a lot closer, and have been asking for documents they haven't in times past. Your organization should be aware of what they want by now.