Hey there,
If I'm understanding it right, you have a dateTime question along with a time question for Start Time and another time question for End Time. You want the two Time questions to be required IF the dateTime question is not filled?
If so, this can definitely be achieved by converting the dateTime question to a string and setting a formula in the required column for the time questions.
If you convert the dasteTime question to a string, when it's blank the value it returns is 0. Thus, we want to set the required condition for the time questions to only be required if the string-length of the dateTime question is 0.
In the "required" column for the two time questions, use the following formula;
string-length(${<dateTime question name>}) = 0
This makes the start and and time questions required when dateTime is blank.
But not required when dateTime is filled
Hope I understood that right! If that needs to be altered for dateTime questions as opposed to just time questions, the same required calculation should still be valid.