For financial data, it would be useful to specify a latency, such that HawkesKernelExp(intensity, decay, latency=0)would be equal to $\phi\left(t\right)=\alpha\cdot\beta\cdot e^{-\beta\cdot (t-t{0})}$ for $t>t{0}$ since the triggering of a new event cannot happen faster than a certain latency (from order book to participant + processing time + from participant to order book); or also because of the granularity of the time stamps.
For financial data, it would be useful to specify a latency, such that
HawkesKernelExp(intensity, decay, latency=0)
would be equal to $\phi\left(t\right)=\alpha\cdot\beta\cdot e^{-\beta\cdot (t-t{0})}$ for $t>t{0}$ since the triggering of a new event cannot happen faster than a certain latency (from order book to participant + processing time + from participant to order book); or also because of the granularity of the time stamps.