Add datetime macros for constants, for clarity:

#define SECS_PER_DAY  86400
#define USECS_PER_DAY INT64CONST(86400000000)
#define USECS_PER_HOUR    INT64CONST(3600000000)
#define USECS_PER_MINUTE INT64CONST(60000000)
#define USECS_PER_SEC INT64CONST(1000000)