'on about 6.30' vs. 'at about 6.30'

[Stone]

English Error, Elementary Level

ESL/EFL Test #11 "At the Finance Company", question 3

I usually get up on about 6.30 and the very first thing I do is switch on the radio.

(a) get
(b) on
(c) switch

English Error, Elementary Level

ESL/EFL Test #11 "At the Finance Company", answer 3

I usually get up at about 6.30 and the very first thing I do is switch on the radio.

Correct entry: at
The error was: (b) on
_________________________

Dear Sir

why not "on about 6.30 "

thanks a lot

Stone

[Conchita]

The prepositions of time and date at and on are used as follows:

at a time: at 6.30, at midnight, at dawn

on a day/date: on Tuesday, on April 24th