Try this formula in B2

=IFERROR(VALUE(SUBSTITUTE(A2,"@","")),VALUE(SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(A2,"PM"," PM"),"pm"," PM"),"@"," ")))

Format Custom:

m/d/yyyy h:mm AM/PM

A
B
1
2
 5/1/2014 2:15PM
5/1/2014 2:15 PM
3
 5/1/2014 @ 2:00 PM
5/1/2014 2:00 PM
4
 5/1/2014 @ 2:45 PM
5/1/2014 2:45 PM
5
 5/1/2014 @ 8:00 AM
5/1/2014 8:00 AM
6
 5/12/2014 @ 6:00 PM
5/12/2014 6:00 PM
7
 5/12/2014 @ 6:15 PM
5/12/2014 6:15 PM
8
 5/12/2014 @ 9:00 AM
5/12/2014 9:00 AM