I often want to determine if a file is stale based on the date of it's source file. There are two ways to do that. One is with the stat() function while another is with the file test operator '-M'.

Using stat()

use constant MTIME    =>    9;    # File modification time

if ((stat("$BulletinSrc"))[MTIME] > (stat("$BulletinPDF"))[MTIME]) {
    $Text .= " PDF is stale             \"$BulletinPDF\"\n";
    $Errors++;
    }
  else {
    $Text .= " PDF is UP-TO-DATE        \"$BulletinPDF\"\n";
    }
[download]

Using '-M'

if (-M "$BulletinSrc" < -M "$BulletinPDF") {
    $Text .= " PDF is stale             \"$BulletinPDF\"\n";
    $Errors++;
    }
  else {
    $Text .= " PDF is UP-TO-DATE        \"$BulletinPDF\"\n";
    }
[download]

Notice that in one 'if' statement the comparison is greater than while the other is less than. That's because stat() returns the date of the file while '-M' returns the length of time since the file was modified.

                  Start of program
  TDS             :
  :               :
  V               V
  |<----- A ----->|
  |---------------:
                  :
      TDS         :
      :           :
      V           :
      |-----------:
      |<--- B --->|

   length(A) < length(B) means A is stale
   date(A)   > date(B)   means A is stale  (notice start of program is
+ irrelevant!)

i.e., stat() returns a TDS (time date stamp)
      -M returns a length since the start of the program
[download]

For me, I have struggled with this until I understood the details.

I hope this helps.

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