Hi TomRO,
Welcome to Chandoo.org forums. I do not have much experience with time and date so I am just attempting one idea. See if this works for you assuming you have data in B2 and A2 and B2 having larger date:
=TEXT(LEFT(TEXT(B2-A2,"[hh]:mm"),FIND(":",TEXT(B2-A2,"[hh]:mm"))-1)/24,"00")&":"&TEXT(MOD(LEFT(TEXT(B2-A2,"[hh]:mm"),FIND(":",TEXT(B2-A2,"[hh]:mm"))-1),24),"00")&RIGHT(TEXT(B2-A2,"[hh]:mm"),3)