No overstay in the OP' case :)

2 separate dates:

1. Visa expiration date: [15 May 2017] the date in which the visa expires. So to use that specific visa, you must enter on or before the expiration date listed on the visa.
2. Permission to Stay date: [17 May 2017] The date in which the entrant must physically exit the country via border immigration checkpoint (or extended at a local immigration office). Usually the permission to stay stamp will have both the entry date and the exit by date stamped or printed in the passport and/or on some type of arrival/departure slip affixed to the passport.

Permission to stay is what counts after you enter any country....

OP must leave Thailand before 23:59:59 on 17 May 2017 to avoid overstay charges. Most land borders close before that time....local immigration offices typically work 08:00-17:00; don't wait until the last second :D

Related FYIs

1) The easiest way to remember this is that you usually acquire a visa BEFORE entering a country. The permission to stay is acquired when you are stamped in by border immigration OR extended in-country at designated gov't [immigration] office.

2) If you hear cases where someone is doing a so-called border run or visa run, it is usually because they either:

• have a multiple entry (ME) visa,
• entering under the Visa Exempt scheme, or
• are acquiring a new visa from a [nearby] Thai Embassy/Consulate.

In either case, technically they are exiting the country (Thailand) >> entering and exiting a different country >> entering the desired country (Thailand) again. When it is all done on the same day, it is casually known as an Out-In entry. (You go out and come back in the same day)

3) Bonus: Some Thai international airport border may waive overstay fine if you are only overstay < 24 hours. I would not recommend relying on that though.