Time-frequency analysis is a modern branch of harmonic analysis. It uses translations and modulations (multiplication by an exponential) for the analysis of functions and operators. It is a form of local Fourier
analysis treating time and frequency simultaneously and symmetrically. The subject is motivated by applications in signal analysis and quantum mechanics.
An introduction to the subject is the book: Foundations of Time-Frequency Analysis by Karlheinz Grochenig 2001.