There are most certainly more elegant ways to do this(!), but just showing that this is possible with a custom event property.
As proof here, I used this formula, which returns to outside if the hours are outside of 8 and 18 (adjust for your timezone).
IFS(
(NUMBER(MID(STRING(@"""{"label":"Time","propertyDefaultType":"datetime","resourceType":"event","type":"datetime","value":"$time"}"""@),12,2))<8),"OUTSIDE",
(NUMBER(MID(STRING(@"""{"label":"Time","propertyDefaultType":"datetime","resourceType":"event","type":"datetime","value":"$time"}"""@),12,2))>18),"OUTSIDE",
)