[Qgis-user] Query cross-table "similar" time stamps?

[Bernd Vogelgesang]

Hi folks,
I'm sure there must be some "easy" solution, but hours of searching
provided no examples I could learn from to develop a solution:

I have a point layer of bird observations and a GPS-track recorded while
observing the birds in an area.
For the interpretation of the findings it would be good to easily find
out, from which position the recorded observation was taken.

My idea was to compare the timestamp of the bird point with the
timestamps of the track points, but I run into huge obstacles right at
start:
I know that I "could" query across layers by the expression aggregate().
Unfortunately, the docs on that is so sparse, that I have not the
slightest idea how to even start with that
(https://docs.qgis.org/testing/en/docs/user_manual/working_with_vector/expression.html?highlight=expression#aggregates-functions)

Ok, in case I could find out how to properly use aggregate, how do I
best compare those time stamps?
The layers derive from gpx-files and QGIS identifies the time tag as
QDateTime.

As it is not very likely that a bird observation was recorded at the
exact same time as a track point was generated, there is no way to
actually compare the values, cause the bird time value will always lie
in between two track point values.
What kind of query can be used to identify those two trackpoints ?

In short: How can I identify/highligt/mark track points that have a
timestamp close the selected point in another layer?

I already created a question on this on gis stackexchange with a
screenshot,but no reactions so far.
https://gis.stackexchange.com/q/30708/8202

Thankful for any hint
Bernd