Old spreadsheets have wrong time and timezone .getValue() from cell - google-apps-script

Question

Problem: wrong time and timezone .getValue() from cell with format time
The bug occurs in old spreadsheets from 2017, but not a spreadsheet from 2022 january or new spreadsheets. update: it did appear in a new sheet as well
What steps will reproduce the problem?
Spreadsheet settings -> timezone GMT+1 (your current timezone)
in cell A1 write 20:00:00
set format cell A1 to "time"
execute this function in google apps script
function showTimeInCellA1() {
const date = SpreadsheetApp.getActiveSheet().getRange("A1").getValue();
SpreadsheetApp.getUi().alert("date in A1 is " + date);
}
Problem: it will alert "date in A1 is Sat Dec 30 1899 19:09:21 GMT+0009"
Expected: I expected time 20:00:00 and GMT+1(because settings spreadsheet are GMT+1)

Answer 1

This appears to be a bug!
Leaving this here for future readers as I think this may have been reported by OP.
There is already a report on Google's Issue Tracker which detail the same kind of behaviour:
wrong time and timezone .getValue() from cell with format time
Google does seem to know about this issue but you can also hit the ☆ next to the issue number in the top left of the page so to let Google know more people are encountering the behaviour so it is more likely to be seen to faster.

Answer 2

A workaround could be to add some years and use the time value of the new date object. I did not examine the exact number of years to be added in order to get valid time information - just tried to get past 1970 which worked for me.
var val2=new Date(val).getTime()+1000*60*60*24*365.25*72 // add ~72 years
Logger.log(new Date(val2))
The value of the current cell in Google Sheets is "8:00:00 AM"
A helper function was used to add the time to a given date:
function utils_buildDate(date,time){
var d=new Date(date);
// +-----------------------------------------------------------------
// | fix time value problem (no date is given in sheet)
// | https://issuetracker.google.com/issues/230650549?pli=1
time=new Date(time).getTime()+1000*60*60*24*365.25*72 // add ~72 years
// +-----------------------------------------------------------------
d.setHours(new Date(time).getHours());
d.setMinutes(new Date(time).getMinutes());
d.setSeconds(new Date(time).getSeconds());
return d;
}