Wǒ chà yī diǎn wàng le jīn tiān shì tā de shēng rì.
I almost forgot that today was her birthday.
Tips
grammar
差一点 can mean either 'almost did X' or 'almost (narrowly avoided) X' — the intended meaning depends on context. 差一点就成功了 means almost succeeded; 差一点摔倒了 means almost fell down (but didn't). Usually, the outcome did NOT happen.