C#, 46 bytes
s=>System.DateTime.Parse(s).ToString("HH:mm");
Strings are taken in input like 12:00 AM
Doing the same thing but substringing on the output from the default call to ToString
(Assuming a culture of en-GB
) is 3 bytes longer for 49 bytes:
s=>(System.DateTime.Parse(s)+"").Substring(11,5);