Germany
Germans are well-known for being organized and punctual, so it’s little surprise that the usage of the more exact 24-hour clock is widespread in the country. In fact. the first country that many people think of when they hear the question "What countries use 24 hour time?" may be Germany. While some countries will only use the 24-hour clock in official contexts (like government documents or public transit schedules), Germans even use the 24-hour clock when having a casual conversation. Using the 12-hour clock is a sign that someone is not a native of Germany, even if they are speaking German.
France
France is another European country that uses the 24-hour clock exclusively in all contexts. Similar to Germany, people will reference the 24-hour clock even when in a casual conversation. Pretty much the only people who use the 12-hour clock in France are foreigners, especially expatriates from England and the United Kingdom.
Romania
In Romania, the 24-hour clock is used exclusively by both the government and private citizens. This is also the case in Moldova, which is another country that speaks Romanian and has a culture heavily influenced by Romania. The usage of the 24-hour clock in Romanian cultures can be traced back to the Cold War, where Romania was closely allied with the Soviet Union (which used the 24-hour clock).
United Kingdom
The United Kingdom is a unique case when it comes to the usage of the 24-hour clock. Both the 24-hour clock and the 12-hour clock are used. The situation is similar to the mixed usage of both the imperial measurement system and the metric measurement systems.
Most government documents in the United Kingdom reference the 24-hour clock. The 24-hour clock is also used in train schedules and the schedules of other types of public transit. However, most private citizens will reference the 12-hour clock in a casual conversation. Even this usage is inconsistent, as private citizens in the United Kingdom sometimes use the 24-hour clock in casual conversation to avoid misunderstandings. There have been many calls within the country to standardize the time-keeping system, but the government has been reluctant to do so.
Vietnam
Vietnam is one of several Asian countries that make use of the 24-hour clock. Vietnam has used the 24-hour clock since the 19th century, when the French introduced it in their new colony. The 24-hour clock remains in use in Vietnam to the present day, nearly 70 years after Vietnam achieved independence. It is seen as one of the few remaining French influences on Vietnamese government and public services.