Here is a formula you can use. Update the column name to match yours.
IF(OR(T({Stop Time}) = blank(), T({Start Time}) = blank()),
"",
CONCATENATE(
DATETIME_DIFF({Stop Time}, {Start Time}, 'hours'),
":",
MOD((ROUND((DATETIME_DIFF({Stop Time}, {Start Time}, 'minutes')+7.5)/15) * 15), 60)
)
)