A small script that attempts to identify when an http resource was last modified. It achieves that by either using the Last-Modified response header or through binary search on the If-Modified-Since/If-Unmodified-Since request headers.

last_modified_detector.sh

#!/bin/bash

#not sure how useful If-Modified-Since/If-Unmodified-Since would be if Last-Modified header is not available:)

resource=${1?Missing the URL of the resource}
accuracy=${2:-2} #by default, tries to identify the first 2 most significant metrics, the year and month

function replacer {
    local result i pattern replacement
    result=$1
    for i in `seq 2 2 $#`;do
        pattern=$i
        replacement=$((i+1))
        result=${result/"${!pattern}"/"${!replacement}"}
    done
    printf %s "$result"
}

function test_ifmod_since {
    [[ $1 -eq $mod_resp_code_ifmod ]] && echo 1 || echo 0
}

function test_ifunmod_since {
    [[ $1 -eq $mod_resp_code_ifunmod ]] && echo 0 || echo 1
}

user_agent="Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/78.0.3904.108 Safari/537.36"
last_modified=$(curl -H "User-Agent: $user_agent" -o /dev/null "$resource" -v |& grep 'Last-Modified:')

if [[ ! -z $last_modified ]]; then
    last_modified=${last_modified#*, }
    day=${last_modified%% *}
    last_modified=${last_modified#* }
    month=${last_modified%% *}
    last_modified=${last_modified#* }
    year=${last_modified%% *}
    echo "$year $month $day LM"
    exit
fi

old_date="Sat, 1 Jan 2000 00:00:00 GMT"
read DAY YYYY MON DD <<<$(date +'%a %Y %b %d' -d '1 day ago')
current_date="$DAY, $DD $MON $YYYY 00:00:00 GMT"

base_resp_code_ifmod=$(curl -H "User-Agent: $user_agent" -s -o /dev/null -w "%{http_code}" -H "If-Modified-Since: ${old_date}" "$resource")
mod_resp_code_ifmod=$(curl -H "User-Agent: $user_agent" -s -o /dev/null -w "%{http_code}" -H "If-Modified-Since: ${current_date}" "$resource")
[[ $base_resp_code_ifmod -ne $mod_resp_code_ifmod ]] && {
    echo "If-Modified-Since: supported ($base_resp_code_ifmod,$mod_resp_code_ifmod)"
    testing_func=test_ifmod_since
    header="If-Modified-Since"
}

base_resp_code_ifunmod=$(curl -H "User-Agent: $user_agent" -s -o /dev/null -w "%{http_code}" -H "If-Unmodified-Since: ${current_date}" "$resource")
mod_resp_code_ifunmod=$(curl -H "User-Agent: $user_agent" -s -o /dev/null -w "%{http_code}" -H "If-Unmodified-Since: ${old_date}" "$resource")
[[ $base_resp_code_ifunmod -ne $mod_resp_code_ifunmod ]] && {
    echo "If-Unmodified-Since: supported ($base_resp_code_ifunmod,$mod_resp_code_ifunmod)"
    testing_func=test_ifunmod_since
    header="If-Unmodified-Since"
}

[[ -z $testing_func ]] && {
    echo "Non of the headers appear to be supported, trying with If-Modified-Since"
    testing_func=test_ifmod_since
    header="If-Modified-Since"
}


header_template="$header: Mon, %DAY% %MONTH% %YEAR% %HOUR%:%MIN%:%SEC% GMT"
sec=({00..59})
min=({00..59})
hour=({00..23})
days=({01..31})
months=(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec)
years=($(eval echo {2000..$(date +"%Y")}))

var_iter_order=(years months days hour min sec)
pattern_iter_order=(%YEAR% %MONTH% %DAY% %HOUR% %MIN% %SEC%)

declare -A replacements=([${var_iter_order[0]}]=2020 [${var_iter_order[1]}]=Jan [${var_iter_order[2]}]=1)
for i in `seq 3 5`;do 
    replacements[${var_iter_order[$i]}]="00"
done

for i in `seq $accuracy 1 $((${#var_iter_order[@]}-1))`;do
    current_var_name=${var_iter_order[$i]}
    header_template=$(replacer "$header_template" "${pattern_iter_order[i]}" "${replacements[$current_var_name]}")
    unset replacements[$current_var_name]
done

var_iter_order=("${var_iter_order[@]:0:$accuracy}")
pattern_iter_order=("${pattern_iter_order[@]:0:$accuracy}")

for ((i=0;i<${#var_iter_order[@]};i++));do
    current_var_name=${var_iter_order[i]}
    current_var=${current_var_name}[@]
    current_var=("${!current_var}")
    
    pre_ifmod_header=${header_template}
    for j in `seq 1 1 $((${#var_iter_order[@]}-1))`;do
        ((relative_j=(i+j)%${#var_iter_order[@]}))
        replacement_var=${var_iter_order[relative_j]}
        pre_ifmod_header=$(replacer "$pre_ifmod_header" "${pattern_iter_order[relative_j]}" "${replacements[$replacement_var]}")
    done
    
    left=0
    right=$((${#current_var[@]}-1))
    while [[ $left -le $right ]];do
        ((middle=(left+right)/2))
        current_var_element=${current_var[middle]}
        ifmod_header=$(replacer "$pre_ifmod_header" "${pattern_iter_order[i]}" "$current_var_element")
        resp_code=$(curl -H "User-Agent: $user_agent" -s -o /dev/null -w "%{http_code}" -H "${ifmod_header}" "$resource")
        if [[ $($testing_func $resp_code) == 1 ]]; then
            right=$((middle-1))
        else
            left=$((middle+1))
        fi
    done
    replacements[${current_var_name}]=${current_var[$((right>=0?right:0))]}
done

for var_name in "${var_iter_order[@]}";do
    printf "${replacements[$var_name]} "
done

[[ $testing_func == test_ifmod_since ]] && printf IM || printf UM