Programming timer

heys guys, i'm new here i'm sorry if not right way start topic. let me that.

my doubt here folowwing:
i'm using arduino uno , trying program timer it. problem is, don't have other plug-in arduino hardwares, uno board , want program timer many hours of intervals, 18 hours led turned on , many other hours turned off.
i've red function delay() don't know maximum value of milisseconds supports, people said it's not function used long time because can reset , not work anymore until reprogammed.

i don't need else turn on , off light, function delay perfect it, have doubts above.

thanks!

admittedly, don't know how long can use delay function reliably. can use millis(); approximately 50 days , have advantage of still being able complete other tasks while timing event.

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