I have a rule that will display a notification on my phone that includes the %date% variable value. I have noticed that when I receive those notifications on my phone, some of the time, instead of displaying a date string, it actually displays "null".
Can you show a screenshot of the entire rule, including triggers and actions?
That looks like it should work. I was asking in case you had other triggers or things like waits, where values that reflect the trigger events may get set to unexpected values (if the trigger is still what you were expecting).
The only likely culprit this leaves is if you did "Run Rule Actions" manually from the rule setup page. This has no trigger, and so like above, values that refer to such events may not be set or may be "old" (last real trigger, for example). Similar if you "call" this rule from another using the "Run rule actions" action from that other rule.
Otherwise, not sure what to suggest unless "Logs" have any clues (errors for this rule, etc.).