timestamp arithmetics in C function

Hello all,

I am trying to implement a C function that accepts a date ("date" in
the sense of a type of information, not a postgres datatype) as
parameter (among others) and returns a certain point in time. (The
background is to calculate the time of dawn and dusk at the given
date.) Ideally, I would like to accept a timestamp value and return
another timestamp as result. I have implemented the function, but I
would like to ask advice on whether my implementation is the
recommended way to achieve this.

To get started - since this is my first attempt at a C function in
postgres - I implemented a function that accepts the date as three
separate int32 values for year, month, and day and returns the time
of dawn as a float8 for the minutes since midnight (this is what the
implemented algorithm internally returns, anyway):

----
PG_FUNCTION_INFO_V1(dawn_utc);

Datum dawn_utc(PG_FUNCTION_ARGS) {
float8 lat = PG_GETARG_FLOAT8(0);
float8 lon = PG_GETARG_FLOAT8(1);
int32 year = PG_GETARG_INT32(2);
int32 month = PG_GETARG_INT32(3);
int32 day = PG_GETARG_INT32(4);
float8 solar_depression = PG_GETARG_FLOAT8(5);

// postgres-independent computation goes here
float8 dawn_utc = calc_dawn_utc
(lat, lon, year, month, day, solar_depression);

PG_RETURN_FLOAT8(dawn_utc);
}
----

This works fine. However, it would be more convenient if the function
would accept a date or timestamp value and return a timestamp. So I
modified the first part of the function like this, based on code
snippets I found in the postgres source code:

----
PG_FUNCTION_INFO_V1(dawn_utc);

Datum dawn_utc(PG_FUNCTION_ARGS) {
float8 lat = PG_GETARG_FLOAT8(0);
float8 lon = PG_GETARG_FLOAT8(1);
Timestamp timestamp = PG_GETARG_TIMESTAMP(2);
float8 solar_depression = PG_GETARG_FLOAT8(3);

struct pg_tm tt;
struct pg_tm *tm = &tt;
fsec_t fsec;

if (timestamp2tm(timestamp, NULL, tm, &fsec, NULL, NULL) != 0)
ereport(ERROR,
(errcode(ERRCODE_DATETIME_VALUE_OUT_OF_RANGE),
errmsg("timestamp out of range")));

// postgres-independent computation goes here
float8 dawn_utc = calc_dawn_utc
(lat, lon, tm->tm_year, tm->tm_mon, tm->tm_mday,
solar_depression;
----

For the second part of the function, I now have to add the
calculated number of minutes to the date portion of the timestamp
variable. One has to be aware that depending on the geographic
location dawn_utc can possibly be negative or larger than 1440 (i.e.
24h). I am not sure whether I should construct an interval value
from the number of minutes and add that to the timestamp. I have not
figured out how to do this, but decided to calculate a new timestamp
in a more fundamental way:

----
tm->tm_sec = 0;
tm->tm_min = 0;
tm->tm_hour = 0;
Timestamp result;
if (tm2timestamp(tm, 0, NULL, &result) != 0)
ereport(ERROR,
(errcode(ERRCODE_DATETIME_VALUE_OUT_OF_RANGE),
errmsg("timestamp out of range")));

#ifdef HAVE_INT64_TIMESTAMP
/* timestamp is microseconds since 2000 */
result += dawn_utc * USECS_PER_MINUTE;
#else
/* timestamp is seconds since 2000 */
result += dawn_utc * (double) SECS_PER_MINUTE;
#endif

PG_RETURN_TIMESTAMP(result);
----

Again this code is based on what I found in the source code. It
seems to work correctly (at least on my development machine), but I
am wondering whether this is a safe and recommended way to achieve
this result or whether it is considered bad practice to manipulate a
timestamp on such fundamental level.

Thank you for your advice and best wishes,
Lutz