two tenses in one sentence: [forgot to tell you that I will]

[ansonman]

I have made up the sentence below.

(1) Ten days ago, I forgot to tell you that I'll be away next week.

My friends are not native speakers. They think the RULE is that you can never use different tenses in one sentence.

I think my sentence is okay. The reason is that I forgot something in the past and my absence will happen in the future.

Please help me. Thank you.

[Alan]

Technically, 'would' is correct by virtue of tense concord rules.

However, for practical communicative purposes, provided that 'next week' refers to a week that is still future at the time of utterance, 'will' would be considered perfectly acceptable by most users.