Sets the timestamp of the tile in milliseconds since January 1, 1970 GMT.
The timestamp indicates when the information to create the tile was last retrieved from the source. It can be
used together with a TTL in order to determine whether to treat it as expired.
The timestamp of a locally rendered tile should be set to the timestamp of the map database used to render it, as
returned by
org.mapsforge.map.reader.header.MapFileInfo#mapDate. For a tile read from a disk cache, it
should be the file's timestamp. In all other cases (including downloaded tiles), the timestamp should be set to
wall clock time (as returned by
java.lang.System#currentTimeMillis()) when the tile is created.
Classes that implement this interface should call
java.lang.System#currentTimeMillis() upon creating an
instance, store the result and return it unless
setTimestamp() has been called for that instance.