How to convert date with awk - macos

Question

My file temp.txt
ID53,20150918,2015-09-19,,0,CENTER<br>
ID54,20150911,2015-09-14,,0,CENTER<br>
ID55,20150911,2015-09-14,,0,CENTER
I need to replace and convert the 2nd field (yyyymmdd) for seconds
I try it, but only the first line is replaced
awk -F"," '{ ("date -j -f ""%Y%m%d"" ""20150918"" ""+%s""") | getline $2; print }' OFS="," temp.txt
and tried to like this
awk -F"," '{system("date -j -f ""%Y%m%d"" "$2" ""+%s""") | getline $2; print }' temp.txt
the output is:
1442619474
sh: 0: command not found
ID53,20150918,2015-09-19,,0,CENTER
1442014674
ID54,20150911,2015-09-14,,0,CENTER
1442014674
ID55,20150911,2015-09-14,,0,CENTER
Using gsub also could not
awk -F"," '{gsub($2,"system("date -j -f ""%Y%m%d"" "$2" ""+%s""")",$2); print}' OFS="," temp.txt
awk: syntax error at source line 1
context is
{gsub($2,"system("date -j -f ""%Y%m%d"" "$2" >>> ""+% <<< s""")",$2); print}
awk: illegal statement at source line 1
extra )
I need the output to be so. How to?
ID53,1442619376,2015-09-19,,0,CENTER
ID54,1442014576,2015-09-14,,0,CENTER
ID55,1442014576,2015-09-14,,0,CENTER

Answer 1

This GNU awk script should make it. If it is not yet installed on your mac, I suggest installing macport and then GNU awk. You can also install a decent version of bash, date and other important utilities for which the default are really disappointing on OSX.
BEGIN { FS = ","; OFS = FS; }
{
y = substr($2, 1, 4);
m = substr($2, 5, 2);
d = substr($2, 7, 2);
$2 = mktime(y " " m " " d " 00 00 00");
print;
}
Put it in a file (e.g. txt2ts.awk) and process your file with:
$ awk -f txt2ts.awk data.txt
ID53,1442527200,2015-09-19,,0,CENTER<br>
ID54,1441922400,2015-09-14,,0,CENTER<br>
ID55,1441922400,2015-09-14,,0,CENTER
Note that we do not have the same timestamps. I let you try to understand where it comes from, it is another problem.
Explanations: substr(s, m, n) returns the n-characters sub-string of s that starts at position m (starting with 1). mktime("YYYY MM DD HH MM SS") converts the date string into a timestamp (seconds since epoch). FS and OFS are the input and output filed separators, respectively. The commands between the curly braces of the BEGIN pattern are executed at the beginning only while the others are executed on each line of the file.

Answer 2

You could use substr:
printf "%s-%s-%s", substr($6,0,4), substr($6,5,2), substr($6,7,2)
Assuming that the 6th field was 20150914, this would produce 2015-09-14