help in hour grep

i have this script that checks for yesterday date and also specific hour in that

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TZ=`date +%Z`+24 ;a=`date +%Y-%m-%d %k`
cd logs
count=0
for i in DBMaint.log
do

cat $i | grep $a >> file12.txt
done
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here k=hour which i want to set it to 22.how to do that.

actually i want to grep yesterday date and in that 22nd hour ,so how to specify that in below command

TZ=`date +%Z`+24 ;a=`date +%Y-%m-%d %k`
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how to set the desired value in above command for hour