  The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has emerged as the
prime telescope for detecting fast radio bursts (FRBs). CHIME/FRB Outriggers
will be a dedicated very-long-baseline interferometry (VLBI) instrument
consisting of outrigger telescopes at continental baselines working with CHIME
and its specialized real-time transient-search backend (CHIME/FRB) to detect
and localize FRBs with 50 mas precision. In this paper we present a minimally
invasive clock stabilization system that effectively transfers the CHIME
digital backend reference clock from its original GPS-disciplined ovenized
crystal oscillator to a passive hydrogen maser. This enables us to combine the
long-term stability and absolute time tagging of the GPS clock with the short
and intermediate-term stability of the maser to reduce the clock timing errors
between VLBI calibration observations. We validate the system with VLBI-style
observations of Cygnus A over a 400 m baseline between CHIME and the CHIME
Pathfinder, demonstrating agreement between sky-based and maser-based timing
measurements at the 30 ps rms level on timescales ranging from one minute to up
to nine days, and meeting the stability requirements for CHIME/FRB Outriggers.
In addition, we present an alternate reference clock solution for outrigger
stations which lack the infrastructure to support a passive hydrogen maser.
