As Michael suggested, if this change is made, and assuming bug 777856 is fixed, indicator-datetime should request the fixed-width numerals. For the clock to change width four times a day (10:00 AM, 12:00 PM, 1:00 PM, 10:00 PM) would be very slightly awkward, but for it to change 14 times every hour (:01, :02, :10, :11, :12, :20, :21, :22, :31, :32, :41, :42, :51, :52) would be much worse. (And horrid if seconds are turned on.) I've added indicator-datetime here because that change would become relevant only if the font is changed.
As Michael suggested, if this change is made, and assuming bug 777856 is fixed, indicator-datetime should request the fixed-width numerals. For the clock to change width four times a day (10:00 AM, 12:00 PM, 1:00 PM, 10:00 PM) would be very slightly awkward, but for it to change 14 times every hour (:01, :02, :10, :11, :12, :20, :21, :22, :31, :32, :41, :42, :51, :52) would be much worse. (And horrid if seconds are turned on.) I've added indicator-datetime here because that change would become relevant only if the font is changed.