As Du Ya served at Luoyang from 788 to when he was replaced by Dong Jin — i.e., 793 — Zhang's service under him must have been during that period. See Old Book of Tang, vols. 146 [Du's biography] and 145 [Dong's biography]. Moreover, as it was likely that Zhang Hongjing observed a three-year mourning period for Zhang Yanshang's death, this would likely be in 790 or later.
The biographies of the princesses in the New Book of Tang do not list a Princess Deyang; presumably, she eventually was created another title, but because her then-title was not listed among those biographies, it is not completely clear whose daughter she was, although the context would appear to indicate that she was a sister or daughter of Emperor Dezong's. See New Book of Tang, vol. 83.
