  Recently, a first-order differentiator based on time-varying gains was
introduced in the literature, in its non recursive form, for a class of
differentiable signals $y(t)$, satisfying $|\ddot{y}(t)|\leq L(t-t_0)$, for a
known function $L(t-t_0)$, such that $\frac{1}{L(t-t_0)}\left|\frac{d
{L}(t-t_0)}{dt}\right|\leq M$ with a known constant $M$. It has been shown that
such differentiator is globally finite-time convergent. In this paper, we
redesign such an algorithm, using time base generators (a class of time-varying
gains), to obtain a differentiator algorithm for the same class of signals,
with guaranteed convergence before a desired time, i.e., with fixed-time
convergence with an a priori user-defined upper bound for the settling time.
Thus, our approach can be applied for scenarios under time-constraints.
  We present numerical examples exposing the contribution with respect to
state-of-the-art algorithms.
