"I eat at 1:30."
Actually, the meaning of character 饭 is not rice. Duolingo makes a mistake here. But it is understandable, as a full explanation would require some knowledge about the history of Chinese food.
When people eat rice, they do not mill it into rice flour. Instead, they heat the rice grains directly with steam. The food cooked in this way is called 饭. In ancient China. 饭 is the most common way for cooking grains. People made 饭 from millet, barley and even wheat (麦饭). But most of them disappeared in history, only the rice 饭 remains.
Since 饭 was so important for Chinese people, the meaning of this character gradually becomes "meal". In modern Chinese, "吃饭" means "have a meal", instead of "eat the steam-cooked grain".
If what you want to say is exactly "steamed rice", use the word 米饭. There are three words in Chinese that may be translated into "rice" in English:
- 稻: rice, the plant
- 米: rice, the grain
- 米饭: rice, threshed, steamed and ready for eating. Sometimes shortened as 饭.
In chinese grammar, the subject must be placed before the object. This is correct because you've technically said the same thing, but turned 一点半 into the subject, making the english equivalent "at one oclock, I eat." Duolingo seems to change its mind re: when to be picky about keeping the sentence order the same or not
Would 我吃饭在一点半(Wǒ chīfàn zài yīdiǎn bàn) make sense with the extra zai? Or is it incorrect?
Is there no need for an AT character?
Also, I submitted 我吃饭一点半 (Wǒ chīfàn yīdiǎn bàn), I thought word order in mandarin was flexible, any particular reason why this was marked wrong?
First, as I've mentioned in the comment above, in Chinese the adverbial is put before the verb in almost every cases, which is different from English. So 我吃饭在一点半 is ungrammatical.
Second, the character "at" is not required in this sentence, but it is required when you want to express the location of the action (e.g. "at the end of the road").
And finally, the word order in mandarin is probably not that flexible as you may think.