At the beginning of an ics file there is a VTIMEZONE component, with a DTSTART property. The value of this property seems odd. For Thunderbird it is 19700308T020000. For Outlook it is 16010101T020000, which are 3/8/1970 2:00AM, and 1/1/1601 2:00AM, respectively.

Why are these dates set to such odd values? What is the significance of them being so widely different (by 369 years)?

I've checked the RFC https://datatracker.ietf.org/doc/html/rfc5545 and find nothing illuminating this.

Does anyone have an idea?

Thunderbird defines 2 timezones, for standard and daylight saving time, in the .ics file. There is a DTSTART when each one starts. The year is irrelevant because it is a yearly repeat. Mine says: The start of Daylight Saving Time repeats yearly on the 2nd Sunday of the 3rd month (RRULE:FREQ=YEARLY;BYDAY=2SU;BYMONTH=3). In 1970 that happened to be 03/08.

The start of Standard time repeats yearly on the 1st Sunday of the 11th month (RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=11). In 1970 that happened to be 11/01. The fact that those were not the correct dates in 1970 - because the current DST rules started in 2007 - is also irrelevant. It is only used to supply a base date for the repeat.

I don't know what's up with Outlook, but it looks like it does not include information above Daylight Saving Time in the .ics file.