There are several different aspects to your question, so bear with me. Perhaps i come from a different background of scripting as i mostly write scripts which have to run on a multitude of different UNIX systems and Linux systems too. Portability is of the utmost importance for me.

GNU is not a given
I often see the -d/--date option of the date command used. Note that standard-conforming date-commands do NOT have that, so you are limiting your scripts to systems with GNU-date (and, IIRC, FreeBSD-date) installed. If this is OK for you, then so be it, but you should be aware that it is a design decision. If you want to avoid this dependency you might want to look at perderabos datecalc script, here is an example of how to use it, along with a C program you may find useful too.

internal and external date representation
The way UNIX/Linux systems represent the date information is like this: there is an "external" representation, which is what you are trying to work with. But this is only a representation of what is used internally to measure time and this is the "UNIX time" or "epoch". It is a 32-bit-unsigned integer counting the seconds since 0:00, Jan 1st, 1970. Notice that it will overflow in somewhere in 2038.

If you are planning to work with dates and do arithmetic with dates my sugestion is to do it like perderabo: create a layer of scripts to convert everything to/from a common representation (ideally this should be the epoch time because it lends itself well to numeric manipulation/calculation) and then calculate with the resulting integers.

interpreting arbitrary formats
First off: arbitrary formats are exactly that: arbitrary. Their (correct) interpretation is somewhat of a guessing game and every rule you can come up with can be circumvented (or "made not to work") by some outlandish format. I have once tried to create a "time format canonifier" you might want to use as a starting point for your own development. It is not exactly what you want but maybe you can get some ideas from it:

Code:

f_ConvertTime ()
{

typeset -i iRetVal=0
typeset    chTime="$1"
typeset -i iHours=0
typeset -i iMinutes=0
typeset -i iSeconds=0
typeset -i iTimeSecs=0

$chFullDebug
                                                 # correct timestring
chTime="$(print - "$chTime" | sed 's/[^0-9]/:/g')" # 14.15.00 -> 14:15:00
chTime="00${chTime}:00:00"                       # 14 -> 14:00:00
						 # :15 -> 00:15:00:00
if [ "$(print - $chTime | cut -d':' -f1)" != "" ] ; then
     iHours=$(print - $chTime | cut -d':' -f1)
     if [ $iHours -lt 0 -o $iHours -gt 23 ] ; then
	  iRetVal=1
     fi
fi
if [ "$(print - $chTime | cut -d':' -f2)" != "" ] ; then
     iMinutes=$(print - $chTime | cut -d':' -f2)
     if [ $iMinutes -lt 0 -o $iMinutes -gt 59 ] ; then
	  iRetVal=1
     fi
fi
if [ "$(print - $chTime | cut -d':' -f3)" != "" ] ; then
     iSeconds=$(print - $chTime | cut -d':' -f3)
     if [ $iSeconds -lt 0 -o $iSeconds -gt 59 ] ; then
	  iRetVal=1
     fi
fi
(( iTimeSecs = iSeconds + iMinutes * 60 + iHours * 3600 ))

print - $iTimeSecs

return $iRetVal
}

I hope this helps.

bakunin