Finding time interval between two consecutive valley points of a signal

Hello,

I serched in the forum to get realted posts of finding time interval between two consecuitve valleys of an analog signal and was understood that converting analog signal into digital will give the reuired.But I don’twant to convert them into digital signal by giving out some threshold value.

Can someone explain of how to find the time interval between two consecutive valley points of an incoming analog signal(as shown in the attached image) and store them in an array. 

Thanks.