We address the problem of constructing matrix-valued interval observers for estimating state transition matrices for time-varying systems. We provide less conservative estimators than those in recent literature. We cover continuousand discrete-time linear systems, under Metzler or nonnegativity conditions on the coefficient matrices. We show how to satisfy our Metzler conditions after simple changes of coordinates. We illustrate our method using a feedback stabilized underwater marine robotic dynamics with unknown control gains.