Separate execution status of piped commands

You have this command

ls -lrt --time-style=+"%b %d %Y %H:%M:%S" /some/path/*.dat|head -1|tr -s " "|cut -d " " -f 9|date --date - +%s

The problem with this is that date does not expect its parameters via stdin, so you need to split it into two parts. You've already attempted this in the second part of your Question:

var1=$(ls -lrt --time-style=+"%b %d %Y %H:%M:%S" /some/path/*.dat|head -1|tr -s " "|cut -d " " -f 9)
echo "var1='$var1'"    # eg var1='11:37:39'
var2=$(date --date="$var1" +%s)
echo "var2='$var2'"    # eg var2='1429094259'

However, when you run the date command, it's giving you the time (11:37:39 - 1429094259) for today (compare it to the value for var2 with my 11:04:45 - 1429092286 and you'll see they are very similar).

Perhaps stat --format '%Y' /tmp/path/*.dat would be easier?

Some answers to specific questions

Is there any way to get to know the value of echo $? [midway through a pipe]? Not directly, no. However, you could either split the pipeline into two parts with a temporary file to hold the intermediate results, or save off the $? value as you go.

cmd1 | ( cmd2; SS=$?; echo $SS >/tmp/cmd2.ss; exit $SS ) | cmd3
cmd2_status=$(</tmp/cmd2.ss)

How can I do what I want in a single pipeline? You can evaluate the time and pass that to date, something like this

date --date=$( cmd1 | cmd2 | cmd2 ) +%s

Why does date --date=- produce the time at midnight? The date command is trying to do something meaningful with the invalid date "-". It happens that this ends up being interpreted as "midnight today". I wouldn't bank on it, though; if you really want midnight I'd go for an explicit date --date='00:00'

In a command where we have lot of pipes redirecting their outputs further, is there any way to get to know the value of echo $? till half of it's execution or till certain number of pipes, as opposed to the overall status of the whole command's?

In bash, there's a PIPESTATUS variable, which is an array containing the exit status of each command in the most recent pipeline.

$ ls -lrt --time-style=+"%b %d %Y %H:%M:%S" /bin/*|head -1|tr -s " "| \
  cut -d " " -f 9|date --date - +%s
1429070400
$ echo ${PIPESTATUS[@]}
141 0 0 141 0
$ kill -l `expr 141 - 128`
PIPE

This tells us that the ls and cut commands exited with a SIGPIPE, which is expected because their output was not completely consumed by the next command in the pipeline.

In the first unbroken command I'm still getting an output without any error and is not something I'm expecting.

That's because the date command is working as it expects, but, I think, not the way you are expecting.

date --date - does not read the date from stdin; instead it uses - as the date string. I can't see where the meaning of a lone - is documented, but it appears to be the same as 0 or 0000, meaning "midnight of the current day".