"It is 6:16."
6 is ろく but when it is combined with a following word it will sometimes change to ろっ.
In English we also change the sound of a word. For example "an" and "a" would be used depending on the following word. This is also like when we pronounce a "t" as a "d" or when we drop "t" from the ends of our words.
It is difficult to say some sounds after く, so a space is added and it is blended together.
ろっぴき（六匹）= roku + hiki = change to っpiki
ろっぷん（六分）= roku + hun = change to っpun
ろっぱい（六杯）= roku + hai = change to っpai
ろっこ （六個）= roku + ko = change to っko
ろくじ （六時）= roku + ji
ろくとう（六頭）= roku + tou
ろくだい（六台）= roku + dai
Can you see the patterns?
When you have a く sound followed by an "H" sound, the く is changed to a small っ and the "H" is changed to a "P" sound.
When you have two K sounds like く and こ next to each other it is hard to say, so く becomes a っ.
Other words that are similar to the words above (j, t, d) are pretty easy to say after く and don't need to be changed.