Shell Programming and Scripting

d="2010-03-17 20:00:00" 
dn=$(echo $d | tr -d ":- ")
t=$dn
while read s
do
  dd=$(($(tr -d ":- \""<<<$s)-$dn))
  [[ $dd -gt 0 ]] && [[ $dd -lt $t ]] && t=$dd && ss=$s
done < infile
echo $ss
"2010-03-18 21:10:00"

2010-03-21 22:30:00
2010-03-18 21:10:00
2010-03-03 14:42:00
2010-04-28 09:30:10

#!/bin/ksh93 or bash or ...
cat file | while read line
do
    day=$(date -u --date="$line" +"%s")
    ((day-=86400))
    yesterday=$( date -u --date="1970-01-01 $day seconds" '+%Y-%m-%d H:%M:%S'   )
    echo "$line => $yesterday"
done

#!/bin/ksh93
cat file | while read line
do
    day=$(printf "%(%#)T" "$line")
    ((day-=86400))
    yesterday=$( printf "%(%Y-%m-%d %H:%M:%S)T \n" "#$day"   )
    echo "$line => $yesterday"
done