The earlier discussion on HMS and that HP doesn't display it in a easy to read format got me thinking about the alpha cababilities on the 42S (and likewise the 41C).
I wrote a quick program that displays HMS as 20 H 10 M 30.12 S and allows for addition and subtraction. Enter the time as HH.MM.SSSS, execute HMS and then enter times to add or substract. It seems to work for positive times. Negative times don't diplay real well.
The display could be changed to ' and " for minutes and seconds.
Note:  is Alpha Append.
LBL "HMS"
XEQ 01
LBL 00
CLMENU
"ENTER"
KEY 1 GTO 01
"+"
KEY 2 GTO 02
""
KEY 3 GTO 03
MENU
STOP
GTO 00LBL 01
CLA
ENTER
AIP
 " H "
FP
100
X
AIP
 " M "
FP
100
X
AIP
 "."
FP
100
X
AIP
 " S "
RDN
AVIEW
RTNLBL 02
>HR
X<>Y
>HR
X<>Y
+
>HMS
GTO 01LBL 03
>HR
X<>Y
>HR
X<>Y

>HMS
GTO 01