Changes to Tooltip.php not taking effect
[[=dateDisplay]] exists in both /wp-content/plugins/the-events-calendar/src/views/month/mobile.php and /wp-content/plugins/the-events-calendar/src/views/month/tooltip.php
Referring to this past answer on customizing this might be helpful: https://theeventscalendar.com/support/forums/topic/help-adding-category-to-tooltip-but-with/#post-1080199
Please let me know if you still need assistance after referencing this information.
That past forum thread I linked to explained how to do such customizing… you have to modify the single-event.php file in that same directory, particularly $additional_data so you can use it in the the tooltip.php file.