According to the text:

Every digit is converted to a binary representation. Then, to make the task easier, you will converted every ON ( or 1 ) signal to dash ( - ) and every OFF ( or 0 ) signal to dot ( . ) so a dash (-) means 'on' and a dot(.) means 'off' then we have the following

1248
-...

that should equal 1... but in the asserts....

assert checkio( "10:37:49" ) == ".- .... : .-- .--- : -.. -..-", "First Test"

10 should be encoded as such then...

first, the 1...

#12
 -.

then the 0...

#1248
 ....

yet the asserts have it wrong...

#12
 .- #would be 2...

even if 10 were referring to 10pm... that would be 22, not 20.