r.sunmask can be used in two ways:

a) define sun position:
r.sunmask elev=dgm25 output=dgm25.shadow alt=47.394203 azim=138.604904

b) define location, date, time
r.sunmask -v el=dgm5 out=shadow year=1997 month=9 day=25 hour=11 minute=15 \
          sec=0 timezone=1 east=3570720 north=5766095

or (using map center coordinates)
r.sunmask -v el=dgm5 out=shadow year=1997 month=9 day=25 hour=11 minute=15 \
          sec=0 timezone=1

Calculating sun position... (using solpos (V. 11 April 2001) from NREL)
 1997.09.25, daynum 268, time: 11:15:00
 long: 10.030458, lat: 52.025555, timezone: 2.000000
 Solar position: sun azimuth 145.121445,
   sun angle above horz.(refraction corrected) 31.662413
 Sunrise time (without refraction): 07:16
 Sunset time  (without refraction): 19:07
Calculating shadows from DEM...   

--------------------------

Since the solpos algorithm expects lat/long coordinates, the module
transforms the local coordinates to lat/long on-the-fly

--------------------------
daylight savings: Rather than convert time to GMT, the solpos
algorithm uses what is called Local Standard Time, which is generally
defined politically as an offset from GMT.  So the key is the offset from
GMT, which the solpos Time Zone parameter.  If the user specifies clock time
(different for winter and summer), he/she would have to change the Time Zone
parameter seasonally in r.sunmask (timezone parameter).


--------------------------

solpos questions can be directed to
> Steve Wilcox (Stephen_Wilcox@nrel.gov)
> National Renewable Energy Laboratory  
> 1617 Cole Boulevard, Golden, CO 80401-3393
> voice: 1-303-275-4061; fax: 1-303-275-4675
  http://rredc.nrel.gov/solar/codes_algs/solpos/