T-SQL: Rounding Time in to 15 minutes but only after 5 minutes past the 15min - sql

Question

I have a table with a datetime field. I'm trying to round up/down the time in 15 minute intervals. But with non standard mathematical rounding rules where rounding up happens if its greater than 5 minutes past the 15 minute interval.
For example
IF 06:05 round down to 06:00
IF 06:06 round up to 06:15
IF 06:20 round down to 06:15
IF 06:21 round up to 06:30
and so on..
I've managed to find here T-SQL: Round to nearest 15 minute interval to round the nearest 15 minutes but this uses mathematical rounding meaning 06:07 would still round down to 06:00 instead of rounding up to 06:15.
Below code where i've got to:
cast(datepart(hour, getdate()) + datepart(minute, getdate()) / 60.00 as decimal(5, 2))

Answer 1

Just use a couple of date tricks.
This code will give you the top of the hour for the time you're evaluating (minutes effectively removed by adding up the hours since the 0 date in SQL):
select dateadd(hour, datediff(hour, 0, getdate()), 0)
From there, you need a CASE expression to evaluate which quartile of the hour the time in question falls into (just a snippet here):
case
when datepart(minute, dtm) > 50 then 60
when datepart(minute, dtm) > 35 then 45
when datepart(minute, dtm) > 20 then 30
when datepart(minute, dtm) > 5 then 15
else 0
end
Put those two pieces together with a DATEADD to decide how many minutes we're adding to that even hour mark:
declare #dtms table (dtm datetime);
insert #dtms (dtm)
values ('2019-07-16T12:05:00'),
('2019-07-16T12:06:00'),
('2019-07-16T12:21:00'),
('2019-07-16T12:29:00'),
('2019-07-16T12:35:00'),
('2019-07-16T12:38:00'),
('2019-07-16T12:56:00')
select
dtm,
dateadd(minute,
case
when datepart(minute, dtm) > 50 then 60
when datepart(minute, dtm) > 35 then 45
when datepart(minute, dtm) > 20 then 30
when datepart(minute, dtm) > 5 then 15
else 0
end, dateadd(hour, datediff(hour, 0, dtm), 0)) as rounded
from #dtms;
Results:
+-------------------------+-------------------------+
| dtm | rounded |
+-------------------------+-------------------------+
| 2019-07-16 12:05:00.000 | 2019-07-16 12:00:00.000 |
| 2019-07-16 12:06:00.000 | 2019-07-16 12:15:00.000 |
| 2019-07-16 12:21:00.000 | 2019-07-16 12:30:00.000 |
| 2019-07-16 12:29:00.000 | 2019-07-16 12:30:00.000 |
| 2019-07-16 12:35:00.000 | 2019-07-16 12:30:00.000 |
| 2019-07-16 12:38:00.000 | 2019-07-16 12:45:00.000 |
| 2019-07-16 12:56:00.000 | 2019-07-16 13:00:00.000 |
+-------------------------+-------------------------+