"Time uses a special time_t variable type, which is the number of seconds elapsed since 1970. Using time_t lets you store or compare times as a single number, rather that dealing with 6 numbers and details like the number of days in each month and leap years."
To get yesterday, subtract the number on seconds in a day from now().
Refer to:
https://www.pjrc.com/teensy/td_libs_Time.html
.