Google App Script code to import events from a Calendar (.ics) file into Google Sheets.

ICS Import.gs

function onOpen() {
  SpreadsheetApp.getUi().createAddonMenu().addItem('Upload', 'upload').addToUi();
}

function upload() {
  SpreadsheetApp.getUi().showModalDialog(HtmlService.createHtmlOutput(
    '<form><input type="file" name="file" onchange="var reader = new FileReader(); reader.onloadend = (e) => google.script.run.withSuccessHandler(google.script.host.close).importICS(e.target.result); reader.readAsText(this.parentNode.file.files[0]);"></form>'
  ), 'Select .ics file');
}

function importICS(form) {
  var inputFields = ['DTSTART', 'DTEND', 'SUMMARY'], // add to this to add more fields
      data = [['Start', 'End', 'Summary']], // should correspond to inputFields
      reDateForDateFunction = /^(\d{4})(\d{2})(\d{2})T(\d{2})(\d{2})(\d{2})Z$/g,
      lines = form.split(/\r\n|\n|\r/);
  for (i in lines) {
    var lineData = lines[i].split(':'), col = inputFields.indexOf(lineData[0])
    if(lines[i] === "BEGIN:VEVENT") {
      data[data.length] = [];
    }
    else if(col !== -1) { // exists in inputFields
      data[data.length - 1][col] = (lineData[0].substr(0, 2) === 'DT') ? new Date(lineData[1].replace(reDateForDateFunction, "$1-$2-$3T$4:$5:$6Z")) : lineData[1]
    }
  }
  SpreadsheetApp.getActiveSpreadsheet().getActiveSheet().getRange(1, 1, data.length, data[0].length).setValues(data)
}

@Herriaan Yes indeed. I'll post it below, but i'd recommend not using this for syncing Goolge Calender with Google Sheet, especially not your private ones. Instead you should use the built in ability of Apps Script to directly read and write events in Google Calender. Works better and is more secure (you don't need to publish an .ics of your calender)
See documentation of the Calendar-Service

This following function downloads an .ics-file that is publicly available under a known domain, and hands it to the function importICS(form):

function downloadICS() {

  var fileURL = 'https://.../calendar.ics';

  var response = UrlFetchApp.fetch(fileURL, {muteHttpExceptions: true}) ;
  var rc = response.getResponseCode()
  if (rc == 200) {
    importICS(response)
  }
    
}

Hi Tom et al, this is an incredibly useful script which i have working. I can't work out how to display the start and end time of each appointment though. I guess it is $4 and $5 but I can't work out how to display it. Any help much appreciated. Thanks very much

While I still can't display the time properly, in case it helps anyone else, I had to change Line 15 to lines = form.toString().split(/\r\n|\n|\r/); to avoid an error message. Brilliant script, i'm using it to push data to a web portal

There where some errors in the code above, like for example when date is including TimeZone or when a FIELD is not existing on the ics file.
I have done some corrections and this is my working code. It allows not only to upload a .ics file from your computer but also import it from a URL (that has to be obtained from your Google calendar private url).
`

function onOpen(e) {
  SpreadsheetApp.getUi().createAddonMenu()
  .addItem('Upload file', 'upload')
  .addItem('Open file from URL', 'downloadICS')
  .addToUi();
  //downloadICS()
}

function downloadICS() {
  var fileURL = 'https://calendar.google.com/....................../basic.ics';
  var response = UrlFetchApp.fetch(fileURL, {muteHttpExceptions: true}) ;
  var rc = response.getResponseCode()
  if (rc == 200) {
    importICS(response.getContentText())
  }
}

function upload() {
  SpreadsheetApp.getUi().showModalDialog(HtmlService.createHtmlOutput(
    '<form><input type="file" name="file" onchange="var reader = new FileReader(); reader.onloadend = (e) => google.script.run.withSuccessHandler(google.script.host.close).importICS(e.target.result); reader.readAsText(this.parentNode.file.files[0]);"></form>'
  ), 'Select .ics file');
}

function importICS(form) {
  var inputFields = ['DTSTART', 'SUMMARY', 'DESCRIPTION'], // add to this to add more FIELDs
      data = [['Inicio', 'Resumen', 'Descripcion']], // should correspond to inputFields
      lines = form.split(/\r\n|\n|\r/);
  for (i in lines) {
    var lineData = lines[i].split(':'), col = inputFields.indexOf(lineData[0].split(';',1)[0])
    if(lines[i] === "BEGIN:VEVENT") { //Adds a new row and initialize it to avoid issues when not found
      data[data.length] = ["","",""]; //If a FIELD is not present on the file, it makes an error: Better to initialize!
      // Its length should correspond to inputFields. Eg: for 7 read fiels --> = ["","","","","","",""]; 
    }
    else if (lines[i].substr(0, 1) === ' ') {
      //If row starts with just SPACE instead of a FIELD, it must be added to previous line as it belows to a multiple line FIELD
      // https://datatracker.ietf.org/doc/html/rfc5545#section-3.3.11
      data[data.length - 1][previous_col] = data[data.length - 1][previous_col] + lines[i]
    }
    else if(col !== -1 && data.length > 1) { // exists in inputFields, and BEGIN:VEVENT found
      //Browser.msgBox("Dealing with: /////////0:" + lineData[0] + " //////////////1: " +  lineData[1] )
      // Date (YYYY,MM,DD) Thus: lineData[1].substr(0, 4) is YYYY, lineData[1].substr(4, 2)-1 is MM (from 0-11) and the other is DD. And last, lineData[1] is the text DATA to import when it is not a Date (DTStart or DTend)
      data[data.length - 1][col] = (lineData[0].substr(0, 2) === 'DT') ? new Date(lineData[1].substr(0, 4),lineData[1].substr(4, 2)-1,lineData[1].substr(6, 2) ) : lineData[1]
      
      for (var j=2; j<lineData.length ; j=j+1) { //It might happen that someone added ':' to the FIELD and therefore it was split too 
        data[data.length - 1][col] = data[data.length - 1][col] + ":" + lineData[j]
      }
      var previous_col = col //To remember the last FIELD saved
    }
  }
  Browser.msgBox( "Total imported lines: " + data.length + " Columns: " + data[0].length )
  SpreadsheetApp.getActiveSpreadsheet().getActiveSheet().getRange(1, 1, data.length, data[0].length).setValues(data)
}

`

Another version that avoid usage of set values to allow the output to be inserted into the context of the command.

usage: =insertICS(".../basic.ics")
:

function icsToTable(icsString) {
  var inputFields = ['DTSTART', 'SUMMARY', 'DESCRIPTION'], // add to this to add more FIELDs
    data = [['Start', 'Summary', 'Description']], // should correspond to inputFields
    lines = icsString.split(/\r\n|\n|\r/);
  for (i in lines) {
    var lineData = lines[i].split(':'), col = inputFields.indexOf(lineData[0].split(';', 1)[0])
    if (lines[i] === "BEGIN:VEVENT") { //Adds a new row and initialize it to avoid issues when not found
      data[data.length] = ["", "", ""]; //If a FIELD is not present on the file, it makes an error: Better to initialize!
      // Its length should correspond to inputFields. Eg: for 7 read fiels --> = ["","","","","","",""]; 
    }
    else if (lines[i].substr(0, 1) === ' ') {
      //If row starts with just SPACE instead of a FIELD, it must be added to previous line as it belows to a multiple line FIELD
      // https://datatracker.ietf.org/doc/html/rfc5545#section-3.3.11
      data[data.length - 1][previous_col] = data[data.length - 1][previous_col] + lines[i]
    }
    else if (col !== -1 && data.length > 1) { // exists in inputFields, and BEGIN:VEVENT found
      //Browser.msgBox("Dealing with: /////////0:" + lineData[0] + " //////////////1: " +  lineData[1] )
      // Date (YYYY,MM,DD) Thus: lineData[1].substr(0, 4) is YYYY, lineData[1].substr(4, 2)-1 is MM (from 0-11) and the other is DD. And last, lineData[1] is the text DATA to import when it is not a Date (DTStart or DTend)
      data[data.length - 1][col] = (lineData[0].substr(0, 2) === 'DT') ? new Date(lineData[1].substr(0, 4), lineData[1].substr(4, 2) - 1, lineData[1].substr(6, 2)) : lineData[1]

      for (var j = 2; j < lineData.length; j = j + 1) { //It might happen that someone added ':' to the FIELD and therefore it was split too 
        data[data.length - 1][col] = data[data.length - 1][col] + ":" + lineData[j]
      }
      var previous_col = col //To remember the last FIELD saved
    }
  }
  return data
}

async function download(fileURL) {
  let response = UrlFetchApp.fetch(fileURL, { muteHttpExceptions: true });
  let rc = await response.getResponseCode()
  console.log(rc)
  if (rc == 200) {
    return response.getContentText()
  }

}

async function insertICS(url) {
  let content = await download(url)
  return icsToTable(content)

}