According to the words of Gjuro Szabó, there used to be a sundial on the south facade of the church, right above the south entrance. In the photo from 1862, you can really notice something like a vertical sundial in that exact place.

1. Szabó, Gjuro (1923) "Zagreb once and Zagreb now", Narodna starina, Vol. 2, No. 4, 73-78. link