Collins Chinese Dictionary Plus (3rd edition), 2011:
一旦 yīdàn
I 名
single day
- 这(這)座城堡竟然在一次大火中毁(毀)于(於)一旦。 Zhè zuò chéngbǎo jìngrán zài yī cì dà huǒ zhōng huǐ yú yīdàn. The great fire destroyed this castle in a single day.
II 副
- 一旦发(發)生核(覈)战(戰)争(爭),整个(個)地球将(將)会(會)陷入灾(災)难(難)。 Yīdàn fāshēng hézhànzhēng,zhěnggè dìqiú jiāng huì xiànrù zāinàn. If atomic war were to break out, it would be disastrous for the whole world.