Compare two Timestamps RTClib.h adafruit

I'm using an RTC DS3231 and with that the RTClib.h Library by Adafruit.
My problem is that I need to compare two Timestamp objects(Meaning produce a Timestamp in the past and when the time comes, compare it two the current Time)
I need the difference in Time in any one unit(minutes, seconds) doesn’t really matter, would prefer minutes though, to calculate with that number.
E.g.: at 18:00:05 on 01.01.2020(DD.MM.YYYY) I created a Timestamp and at 19:00:00 on the 02.01.2020 I want to know the time passed from the given Timestamp on out. The value that I need would be something like 1499 because 1499 minutes passed between these timestamps.
How would I do this? are there any functions like:
difference(minutes, Timestamp1, Timestamp2)

also: will the Arduino save these Timestamp objects for when it has no power? The RTC has a separate battery.
E.g.: The Arduino has power and runs the program. It stores 3 different Timestamps and I turn the power off. When I turn the power on again, will I be able to use these Timestamps again?

The normal way to compare times is to have both in a common format such as Unix time. Where are your timestamps coming from ?

will the Arduino save these Timestamp objects for when it has no power?

Not unless you explicitly save them to non volatile memory and retrieve them when needed.

Search for DS3231 EEPROM

are there any functions like: difference(minutes, Timestamp1, Timestamp2)

See TimeLib.h and related libraries.