Abstract: This study investigates how symptoms of PTSD, cannabis use, and negative emotions vary on a day-to-day basis among US military veterans. Ecological momentary assessment (EMA) was used to gather real-time data from participants multiple times a day, providing a nuanced understanding of how psychological states and behaviours shift in close proximity to each other. MAVERICK (built on the Ration framework), a custom-built app developed to collect high-frequency, in-the-moment data from users was used. MAVERICK has been designed to integrate with digital devices and wearables, offering a scalable way to track mental health and behavioural patterns over time. This study showcases how MAVERICK can support mental health research through reliable, user-driven data collection outside of traditional clinical environments. The findings suggest that on days when veterans experienced more severe PTSD symptoms, they were more likely to use cannabis and report greater negative affect. Importantly, the study moves beyond cross-sectional assumptions to highlight the dynamic interplay between mental health symptoms and substance use in everyday life. This work adds to a growing body of research using digital tools to better understand mental health in real-world contexts. It also demonstrates how apps like Maverick can support longitudinal behavioural health monitoring, particularly in populations with complex needs.