function "dtutils.string.substitute()" fails when no time data is provided in Exif data

Start in lua/lib/dtutils/string.lua by reading build_substitute_list and exiftime2systime, then run the reported substitution script against an image with no EXIF time data. Done means the default timestamp is accepted without an os.time error on the affected system.

When running a script using "dtutils.string.substitute()" function on a file without any time data in Exif, the "dtutils.string.substitute()" function fails.

190.9832 LUA ERROR : ...\AppData\Local\darktable/lua/lib/dtutils\string.lua:748: time result cannot be represented in this installation
stack traceback:
	[C]: in ?
	[C]: in function 'os.time'
	...AppData\Local\darktable/lua/lib/dtutils\string.lua:748: in upvalue 'exiftime2systime'
	...AppData\Local\darktable/lua/lib/dtutils\string.lua:843: in function 'lib/dtutils.string.build_substitute_list'
	...AppData\Local\darktable/lua/lib/dtutils\string.lua:1298: in function 'lib/dtutils.string.substitute'

After looking at the "string.lua" code, it seems that "build_substitute_list" function tries to use "0000:00:00 00:00:00" as default value if the exif time is not available, but the "os.time{}" conversion called later through "exiftime2systime(datetime_taken)" does not like it. The minimum time value accepted on several systems is January 1st, 1970. I updated the "string.lua" code on my installation (replacing "0000:00:00 00:00:00" with "1970:01:01 00:00:00") and this fixes the issue.

Code from string.lua :

local function exiftime2systime(exiftime)
  local yr,mo,dy,h,m,s = string.match(exiftime, "(%d-):(%d-):(%d-) (%d-):(%d-):(%d+)")
  return(os.time{year=yr, month=mo, day=dy, hour=h, min=m, sec=s})
end

[...]

function dtutils_string.build_substitute_list(image, sequence, variable_string, username, pic_folder, home, desktop)
[...]
  if image.exif_datetime_taken and image.exif_datetime_taken ~= "" then
    datetime_taken = image.exif_datetime_taken
  else
    if use_millisecs then
      datetime_taken = "0000:00:00 00:00:00.0"
    else
      datetime_taken = "0000:00:00 00:00:00"
    end
  end
[...]

I see this issue on Windows 11, DarkTable 5.4.0. Other systems could behave the same, or not.