如何去记录一个时间点的数据？

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i  met some problems while programing.i want to record a data (like the time) when the enviroment is dark enough,
such as  when the lx is less than 100,it can record the time right now,and when lx less than 100 again, it can record another time again,too
but i dont know how to do it exactly.
it would be appreciate if you can give me some suggesttions .