I'd guess the Time column starts out as character data.
dt:Time << Data Type( Numeric, Format( "h:m", 12 ), Input Format( "h:m" ) )
The reason the (copied from scripting index...) <<DataType message takes additional parameters after Numeric vs Character is because converting a character column of dates to numeric needs the informat to get the right answer. So yes, you should provide both format and informat.
The same thing is true in the dialogs for converting a character column of dates to a numeric column of dates: if you don't supply the informat, the date strings get converted to missing values.
We are also guessing. If you copy the error message from the log we might guess better. As Jim hinted, we also don't know what the data type of your column is, before you try to change its type.
Craige