Woody, no one really knows yet, but yes - whatever 'completion' turns out to be, it is supposed to occur by that date.
And tony1851's right - the expiry date for larger home extensions (Class A, Part 1) was been extended in March of this year. Currently, it will expire 30th May 2019.
The development must be completed before 30 May 2019, and the developer must notify the local planning authority of the completion of the development as soon as reasonably practicable after completion. This notification must be in writing and must include the name of the developer, the address or location of the development, a description of the development, including measurements and calculations (by reference to the new size limits) and the date of completion.