Help with time keeping and loops

NOOB QUESTION: Hi everyone . Going nuts trying to figure something out. I have a load sensor on top of a box. Leave your phone on the box for X minutes and the box pops open. So I need to measure the start time, then wait until some time passes, and then open the box while flashing a LED. I can't seem to find how to do it without resetting the start time inside the loop(). this is as far as I got... Anyone?

#include "HX711.h"
#include <Servo.h>

// HX711 circuit wiring
const int LOADCELL_DOUT_PIN = 2;
const int LOADCELL_SCK_PIN = 3;
Servo myservo;  // create servo object to control a servo

HX711 scale;

int led = 13; // the pin the LED is connected to
const int openTime = 10;   // seconds to open box
unsigned long currentMillis; 

void setup() {
// myservo.attach(9);  // attaches the servo on pin 9 to the servo object
  pinMode(led, OUTPUT); // Declare the LED as an output
  digitalWrite(led, LOW); // Make sure LED is off
  Serial.println("HX711 Demo");

  Serial.println("Initializing the scale");

  // Initialize library with data output pin, clock input pin and gain factor.
  // Channel selection is made by passing the appropriate gain:
  // - With a gain factor of 64 or 128, channel A is selected
  // - With a gain factor of 32, channel B is selected
  // By omitting the gain factor parameter, the library
  // default "128" (Channel A) is used here.


void loop() {
  Serial.print("one reading:\t");
  Serial.print(scale.get_units()*2.53, 1);
  Serial.print("\t| average:\t");
  Serial.println(scale.get_units(10), 1);

  if (scale.get_units()*2.53 > 100)
            currentMillis = millis()/1000;
            digitalWrite(led, HIGH); // Turn the LED on  
  scale.power_down();			        // put the ADC in sleep mode

void blink()  
  if (millis()/1000 == currentMillis + openTime) {
  digitalWrite(led, LOW); // Turn the LED off          
  delay(1000);// Wait for 1000 milliseconds (1 second) // 1 h = 3600000 ms
  digitalWrite(led, HIGH); // Turn the LED on  
  delay(1000);// Wait for 1000 milliseconds (1 second) // 1 h = 3600000 ms

Save the start time when the start condition becomes true rather than when it is true

See the StateChangeDetection example in the IDE

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