How to convert in file csv date in specific column to unix date

With GNU sed

sed -E '2,$ s/(("[^"]*",){4})("[^"]+")(.*)/echo \x27\1"\x27$(date -d \3 +%s)\x27"\4\x27/e'

• 2,$ to skip header from getting processed
• (("[^"]*",){4}) first four columns
• ("[^"]+") fifth column
• (.*) rest of the line
• echo \x27\1"\x27 and \x27"\4\x27 preserve first four columns and rest of line after fifth column, along with adding double quotes to result of date conversion
• $(date -d \3 +%s) calling shell command with fifth column value

Note that this command will fail if input can contain single quotes. That can be worked around by using s/\x27/\x27\\&\x27/g.

You can see the command that gets executed by using -n option and pe flags

sed -nE '2,$ s/(("[^"]*",){4})("[^"]+")(.*)/echo \x27\1"\x27$(date -d \3 +%s)\x27"\4\x27/pe'

will give

echo '"58.75","5.33","kg","7","'$(date -d "2020-7-11 19:29:29" +%s)'","5"'

For 58.25,5.89, kg, 7,2020 / 7/12 11:23:46, "5" format, try

sed -E '2,$ s/(([^,]*,){4})([^,]+)(.*)/echo \x27\1\x27$(date -d "\3" +%s)\x27\4\x27/e'

or (adapted from https://stackoverflow.com/a/62862416)

awk 'BEGIN{FS=OFS=","} NR>1{$5=mktime(gensub(/[:\/]/, " ", "g", $5))} 1'

Note: For the sed solution, if the input can come from outside source, you'll have to take care to avoid malicious intent as mentioned in the comments. One way is to match the fifth column using [0-9: -]+ or similar.

Using GNU awk:

$ gawk '
BEGIN {
    FS=OFS=","
}
{
    n=split($5,a,/[-" :]/)
    if(n==8)
        $5="\"" mktime(sprintf("%s %s %s %s %s %s",a[2],a[3],a[4],a[5],a[6],a[7])) "\""
}1' file

Output:

"Weight","Impedance","Units","User","Timestamp","PhysiqueRating"
"58.75","5.33","kg","7","1594484969","5"

With GNU awk for gensub() and mktime():

$ awk 'BEGIN{FS=OFS="\""} NR>1{$10=mktime(gensub(/[-:]/," ","g",$10))} 1' file
"Weight","Impedance","Units","User","Timestamp","PhysiqueRating"
"58.75","5.33","kg","7","1594513769","5"