Question: When was the Austrian pianist which Stefan Knüpfer worked with born 

Context paragraphs:
Josef Dachs: Josef Dachs (30 September 1825 – 6 June 1896) was an Austrian pianist and music teacher born in Regensburg.  He received his music education by Simon Sechter and Carl Czerny, worked as a concert pianist and premiered many of his works.  He became professor at the Vienna Conservatory in 1850.  Among others, he taught Isabelle Vengerova, Hugo Wolf, Ferdinand Löwe, and Russian pianist and composer Josef Rubinstein (1847-1884).  He died in Vienna.

Roland Batik: Roland Batik (born August 19, 1951 in Vienna) is an Austrian pianist, composer, jazz musician and piano teacher.  In the style of his compositions, he is seeking a fusion of classical elements with Jazz.  Among other, his music is frequently broadcast in the Austrian radio program Ö1.

Pianomania: Pianomania is a 2009 German-Austrian documentary film by directors Lilian Franck and Robert Cibis.  The film presents Stefan Knüpfer, a virtuoso piano tuner from the piano company Steinway & Sons, in his work with pianists such as Lang Lang, Alfred Brendel and Pierre-Laurent Aimard.

Ingrid Haebler: Ingrid Haebler (born June 20, 1926 in Vienna, Austria) is an Austrian pianist.  She studied at the Salzburg Mozarteum, Vienna Music Academy, Geneva Conservatory and privately in Paris with Marguerite Long.  She toured worldwide.  She is best known for a series of recordings from the 1950s to 1980s.  Her complete set of Mozart's piano sonatas for the Denon label is still regarded as among the finest sets.  Haebler also recorded all of Mozart's piano concertos (most of them twice), often with her own cadenzas - and all of Schubert's sonatas.  She was one of several Austrian musicians to experiment early with period instruments, having recorded music of J. C. Bach on a fortepiano.  Her recordings of Mozart and Beethoven with violinist Henryk Szeryng are particularly prized.

Stefan Vladar: Stefan Vladar (born in Vienna, 1965) is an Austrian pianist and conductor.

Alfred Brendel: Alfred Brendel KBE (born 5 January 1931) is an Austrian pianist, poet and author, known particularly for his performances of Mozart, Schubert, Schoenberg, and especially Beethoven.

Catherina Cibbini-Kozeluch: Catherina Maria Leopoldina Cibbini-Kozeluch (Katerina Koželuh) (b. 20 February 1785, d. 12 August 1858) was an Austrian pianist and composer of Bohemian ancestry.  She was born in Vienna, the daughter of prominent pianist and music publisher Jan Antonín Koželuh.  She studied music with her father and also with Muzio Clementi.

Grete Trakl: Grete Trakl, full name Margarethe Jeanne Trakl, married name Grete Langen (born 8 August 1891 in Salzburg; died 21 September 1917 in Berlin) was an Austrian pianist and sister of the Austrian poet Georg Trakl.

Alfred Grünfeld: Alfred Grünfeld (Prague, July 4, 1852 – January 4, 1924, Vienna) was an Austrian pianist and composer.  He was the first pianist of note to make genuine commercial recordings, and a CD featuring many of his tracks is available.  His brother was musician Heinrich Grünfeld.  His Viennese house is at Getreidemarkt 10, and has a plaque above the door.  His grave can be visited in the Zentralfriedhof in Vienna.

Karl Ulrich Schnabel: Karl Ulrich Schnabel (August 6, 1909 – August 27, 2001) was an Austrian pianist, and the son of pianist Artur Schnabel and operatic contralto and lieder singer Therese Behr.

Supporting paragraphs: Pianomania Alfred Brendel
Answer: 5 January 1931