Look at the DS1307 registers and format your data accordingly. The values you write should be packed BCD values wherever needed.

eg: To write 21 minutes, you need to write HEX 21 to the minutes register. Same for the other registers like hours and seconds. Only make sure to write to the bits needed by the registers.

I haven't used PBP to write a DS1307 yet, but have some C code that works with the IC. Let me know if you wish to look at it for inspiration