Sun Solaris Unix Shell script that adds 2 to a string numeric and checks last value for change before emailing

Currently our Sun E450 could detect room temperature (from "prtdiag -v" and I've
observed that it's consistently reporting about 1.5-2.0 Celsius lower than the
general DC room temperature :

# prtdiag -v | grep -i ambie
AMBIENT    19

"date" command in this Solaris box's format is :
# date
Sat May  9 06:06:58 SGT 2009


I'll need a Shell script that run every 1 minute (I'll put this into the crontab as
1,2,...59 * * * * /path/script_name) that will do the following :

I'll need this Shell script to be able to notify/email me when room temperature
(>= 21  AND there's a change from the last value)   or
(<= 20  AND the temperature recorded one minute ago is >= 21)

Above script basically notifies me once only (not repeatedly) when temperature
hits 21 C  but if temperature keep going up (at levels above 21 C) it will keep emailing/
notifying me.
This script only notify me once when temperature go below 21 C (but only notify
once and not repeatedly and it will stop notifying if room temperature stays below
20 C)  as the pseudo codes below describe :

Currentdatetime=yyyymmdd:hhmm   (24 hr format)
echo RoomTemp + " " + Currentdatetime  >> yyyymmdd_outputfile
add 2 to the value given by "prtdiag -v | grep AMBIENT" (call this variable RoomTemp
  if RoomTemp equals or exceeds 21
    check RoomTemp vs last_temperature recorded in last line of yyyymmdd_outputfile
        if RoomTemp equal or exceed 21 AND is higher than last_temperature
           email to myemail the value Currentdatetime+" "+":Room temp is now "+RoomTemp
        endif
  if last_temperature in last line of yyyymmdd_outputfile >= 21 and RoomTemp is <=20
    email to myemail the value Currentdatetime+" "+":Room temp now is down to "+RoomTemp
  endif
exit