That is NOT even the correct timetable.. Here is the correct timetable which has the 40min service etc. https://www.firstgroup.com/uploads/node_images/Service_20_63_68_TT_Bklt_WEB 28sept_0.pdf
1728 from the Gyle then, not 1730 if we're really being that pedantic. That was the first result I had off Google which was obviously incorrect by 2 minutes for that service.
It's quite common for passengers to refer a service that's close to 15/30/45 as that time - e.g people will say the 0813 is the quarter past.
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