[times at which] or [times when]

[ansonman]

I have made up the sentences below.

(1a) Jack and I have been working here for twelve years. He works part-time and I work casually. So, most of the time we share this room independently. However, there are times at which we share the backroom at the same time if our work hours match.

(1b) Jack and I have been working here for twelve years. He works part-time and I work casually. So, most of the time we share this room independently. However, there are times when we share the backroom at the same time if our work hours match.

Please help me. Thank you very much.

[Alan]

Both aceptable, but (b) is more conversatuional.