Guess it would help to show what you’re trying to use as a formula.
Here’s one possible formula using sin() and the day as input. Every 100 days it will oscillate between 50 to 90.
70+20*sin(day/100*360)
Alternately you can use cosp() which interpolates between two values using a sine wave.
cosp(50,90, day/100/2)
The /2 was needed otherwise it would be 200 days for a full cycle.