Is the Arduino to be battery powered? If not, why bother going to sleep? The pump will have a power source, yes?
Anyway, as others have said, use the noted output pin of the DS3231 to an interrupt pin on the Arduino to wake it up at the desired Alarm time:
"Active-Low Interrupt or Square-Wave Output. This open-drain pin requires an external pullup resistor
connected to a supply at 5.5V or less. This multifunction pin is determined by the state of the INTCN bit in
the Control Register (0Eh). When INTCN is set to logic 0, this pin outputs a square wave and its frequency
is determined by RS2 and RS1 bits. When INTCN is set to logic 1, then a match between the timekeeping
registers and either of the alarm registers activates the INT/SQW pin (if the alarm is enabled). Because the
INTCN bit is set to logic 1 when power is first applied, the pin defaults to an interrupt output with alarms
disabled. The pullup voltage can be up to 5.5V, regardless of the voltage on VCC. If not used, this pin can
be left unconnected."
Note the part number on your module and select the correct data sheet so you can program it correctly: