I posted awhile ago about the name [name_u]Day[/name_u], and well…no one seemed to like it. That’s fine (I don’t think it would work as a first name, either), but WDYT of it as a nickname for [name_m]Daniel[/name_m]?
I think it would be a good nickname for [name_m]Daniel[/name_m]. Much better than a name on it’s own.
I think it could work, but the Grease fan in me wants [name_m]Daniel[/name_m] to be shortened to [name_u]Danny[/name_u] 
I don’t understand the appeal at all, but any nickname works if you want to use it. It’s just a personal nickname, it doesn’t have to be intuitively linked to the formal name. It seems like you’re the kind of person who thinks it must be intuitively linked… in that case, [name_u]Day[/name_u] seems fine for [name_m]Daniel[/name_m].
However, the connection to actor [name_m]Daniel[/name_m] [name_u]Day[/name_u]-[name_m]Lewis[/name_m] could be an issue.