What timezone should be used for Ottawa?
When I first started working with the Olson tz database I found it a bit confusing because my city was not listed; and I didn't know which city to choose.
Ottawa is the capital city of Canada, and the fourth largest city in Canada (after Toronto, Montreal and Vancouver), with over 1 million in the capital region, but America/Ottawa is not one of the options provided by the Olson zoneinfo tz database.
The tz database, as put together, spells out timezone offsets as well as daylight-saving rules for numerous locations, for now and the past. For each country the largest populated area (not necessarily the capital city) is generally listed, then if there is an area in that country with any differing rules SINCE JAN 1st 1970 the largest populated place in that region also gets a listing, etc.
The case for Ottawa is interesting. Though Ottawa is the capital of Canada, Ottawa is in the province of Ontario. And the capital of Ontario is Toronto, not Ottawa. So those people that see timezones as political may decide that Ottawa should use the 'America/Toronto' timezone.
On the other hand, Ottawa is on the Ottawa river which flows to Montreal. And the main historical business of Ottawa (before it was chosen as capital) was lumbering. Ottawa at one point had the world's largest lumbering operation, and most of the logs were floated down the Ottawa river to Montreal. (Picture) As well, Ottawa is only 100 miles from Montreal - straight down the Ottawa river, whereas it over 220 miles from Ottawa to Toronto - through bushland. So if you look at Ottawa historically, or commercially, or physically, there is no doubt that Ottawa has historically had much closer ties to Montreal than to Toronto: hence 'America/Montreal' in the tz database.
A little more backgrounder...
Montreal was the largest city in Canada until the 1970s, when it was overtaken by Toronto. But as far back as the mid 1950s, Montreal and Toronto decided to keep their clocks in sync. So the question is why are both Toronto and Montreal listed in the database, and can I choose either?
Well, if you are only interested in timezone information since the mid 1950s, you can presumably choose either Toronto or Montreal, but if you are interested in dates before 1957 (when Toronto and Montreal clocks start to diverge), I would say go with Montreal! Without researching the details of Ottawa's own daylight-saving time decisions, it would be most likely that Ottawa's clocks were more in sync with Montreal than Toronto.
Finally, why are both Montreal and Toronto in the database if their clocks have agreed since the 1950s? Well, its because the cities themselves decided to keep their clocks in sync by their own rules, but Canada itself didn't have standardized daylight saving rules until 1974, and at that point the two cities decided to switch from using their own rules to the Canada rules. Its because of that transition, and the fact they made their own rules before 1974, that both cities are in the database.