Foreword: The question was asked on November 30, that's why the month and day part seems like today.
Zero values are parsed properly, but there is no "Month 0". The first month is January which has numeric value 1. Similarly, there is no 0th day of month, the first day in every month is 1.
time.Date documents that:
The month, day, hour, min, sec, and nsec values may be outside their usual ranges and will be normalized during the conversion. For example, October 32 converts to November 1.
So if you pass 0 for month and day, that is interpreted the same as passing 1 to month and day, and adding -1 to each.
See this example:
bday := time.Date(0, time.Month(0), 0, 0, 0, 0, 0, time.UTC)
fmt.Println(bday)
bday2 := time.Date(0, time.Month(1), 1, 0, 0, 0, 0, time.UTC)
fmt.Println(bday2)
bday2 = bday2.AddDate(0, -1, -1)
fmt.Println(bday2)
Which outputs (try it on the Go Playground):
-0001-11-30 00:00:00 +0000 UTC
0000-01-01 00:00:00 +0000 UTC
-0001-11-30 00:00:00 +0000 UTC
So the result becoming "today" is purely by accident, today being November 30. If you run the code tomorrow, the month-day part will not be today anymore but yesterday.
Mere co-incidence, as a workaround I have to wrap the operation inside an if condition, I believe.