I am wondering if there is a way to make sky.rad by using epw file? Is it also possible to fix certain period of time for the sky.rad, e.g. 10 Jul, 7:00 - 12:00; and 24/7/365?
There is a tool, written by Christoph Reinhart and included in Radiance, called epw2wea to convert your EPW file into something a little more concise and easier to read. From this file, you can use the gendaymtx program to create a matrix of sky patch values over the time period covered. If you only want a few time points, you can edit the WEA file and save a version with just those times of interest. Or, you can use the values in the file to run gendaylit manually to create a sky position for the relevant dates & times. I don’t know the exact options you need for that, but perhaps someone else can offer that information. It depends somewhat on the flags at the beginning of the WEA file.
I must point out that people usually use the entire “typical weather year” for a location because if you subsample it too much, pulling out a day here and a time there, you lose the statistical value of starting with these data in the first place.
Finally, you should refer to the excellent research of Eleonora Brembilla, who has studied the measurements upon which such data is based, and in some cases found them to be lacking. (See her presentation from last year’s workshop for example.)