This got lost in the types refactor, and was a rare case of the pointer receiver actually being needed - various apps depended on this function modifying the underlying value.
One could also argue for this being factored out into a NewDateTimeFromTimestamp function or something, but this way is source-compatible with older code.
Also remove StructureInterface, and make all structures initialize all
their fields. The structure header is now added as a common function
`ExtractHeaderFrom` and `WriteHeaderTo`.
