linux kernel: understand how jiffies diff is calculated

I came across the following code in the kernel:

/* * Have the 32 bit jiffies value wrap 5 minutes after boot * so jiffies wrap bugs show up earlier. */             #define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))static inline u32 cstamp_delta(unsigned long cstamp){        return (cstamp - INITIAL_JIFFIES) * 100UL / HZ;}

where cstamp value is in jiffies.

This is the code from net/ipv4/devinet.c where IP address per interface is implemented (among other things).

I see that INITIAL_JIFFIES macro takes the value of 5 minutes (300) and converts it in jiffies (-300*HZ), and typecasting ensures correct value wrapping.

But why is it explicitly set to negative value (-300*HZ)?

I'm not sure, in what units cstamp_delta() returns?