How to apply function by groups in array in R?

I have a 3d array with latitude, longitude and datetime (Year_Month_Day_Hour). Which is the best way in R to apply a function over the array by groups (in this case by years or month or days)? The result should be an array with the mean values. The 3 dimension is than the year, month or day.

str(data)
 num [1:7, 1:7, 1:5] 977 994 1010 1020 1026 ...
 - attr(*, "dimnames")=List of 3
  ..$ : chr [1:7] "60" "57.5" "55" "52.5" ...
  ..$ : chr [1:7] "-30" "-27.5" "-25" "-22.5" ...
  ..$ : chr [1:5] "2014_10_01_00" "2014_10_01_06" "2014_10_01_12" "2014_10_01_18" ...

Example (truncated):

dput(data) structure(c(977.2, 994.4, 1009.8, 1020.1, 1026.4, 1029.4, 1029.2, 
978.7, 995.7, 1010.2, 1020.5, 1026.5, 1028.8, 1028.3, 982, 997.5, 
1011.3, 1021.2, 1026.1, 1027.4, 1027.1, 986.2, 999.9, 1013, 1021.7, 
1025.1, 1025.7, 1026, 990.6, 1002.7, 1014.5, 1021.3, 1023.9, 
1024.7, 1025.6, 995.1, 1005.7, 1015.2, 1019.9, 1022.6, 1024.5, 
1025.9, 999.1, 1008, 1015.1, 1018.6, 1021.8, 1024.5, 1026.6, 
982.1, 998.9, 1011.8, 1020.1, 1025.5, 1028.4, 1028.8, 981.9, 
999.3, 1012.7, 1021.2, 1026.4, 1028.8, 1029, 983.9, 1000.2, 1013.5, 
1022.1, 1027, 1028.9, 1028.9, 987.1, 1001.8, 1014.6, 1022.7, 
1027.3, 1028.6, 1028.2, 990.9, 1004.1, 1016.1, 1023.3, 1027.2, 
1027.9, 1027.4, 995.1, 1006.9, 1017.8, 1023.8, 1026.8, 1027, 
1026.9, 999.5, 1010.1, 1019.1, 1023.8, 1025.9, 1026.1, 1026.9, 
990.3, 1002.3, 1010.9, 1018.3, 1024, 1027.6, 1028.6, 990.6, 1004.1, 
1013.2, 1020.8, 1026.2, 1029.3, 1029.8, 992.1, 1005.5, 1015.2, 
1023, 1028, 1030.4, 1030.5, 994.5, 1007, 1017.2, 1024.7, 1029.4, 
1031, 1030.3, 997.4, 1008.8, 1019, 1025.7, 1030, 1031, 1029.8, 
1000.1, 1010.9, 1020.9, 1026.5, 1030, 1030.6, 1029.5, 1002.9, 
1013.3, 1022.6, 1027.2, 1029.7, 1029.7, 1029.2, 993.6, 997.5, 
1001.3, 1007.4, 1015.5, 1022.7, 1026.4, 996.1, 1001.1, 1005.8, 
1012.7, 1020.1, 1025.6, 1027.9, 998.4, 1004.5, 1010.4, 1017.6, 
1023.8, 1027.6, 1029.1, 1000.2, 1007.3, 1014.4, 1021.5, 1026.4, 
1029, 1029.7, 1002, 1010, 1017.8, 1024.3, 1028.4, 1029.9, 1029.6, 
1004.3, 1012.9, 1020.7, 1026.3, 1029.7, 1030.2, 1029.3, 1006.9, 
1016, 1023.2, 1027.7, 1030.3, 1029.7, 1028.6, 987.9, 989.6, 995.1, 
1002.9, 1010.8, 1018.9, 1025.1, 989.8, 990, 995.1, 1004.7, 1013.9, 
1021.8, 1026.8, 993.1, 992.6, 998.1, 1008.8, 1018, 1024.6, 1028.3, 
996.9, 997.3, 1003.9, 1014, 1021.9, 1026.8, 1029.1, 1000.3, 1003.1, 
1010.5, 1019, 1025.2, 1028.5, 1029.6, 1003.6, 1008.7, 1016.4, 
1023.1, 1027.8, 1029.8, 1029.9, 1007.3, 1013.7, 1020.8, 1026.3, 
1029.8, 1030.2, 1029.6), .Dim = c(7L, 7L, 5L), .Dimnames = list(
c("60", "57.5", "55", "52.5", "50", "47.5", "45"), c("-30", 
"-27.5", "-25", "-22.5", "-20", "-17.5", "-15"), c("2014_10_01_00", 
"2014_10_01_06", "2014_10_01_12", "2014_10_01_18", "2014_10_02_00"
)))

SOLUTION:

group <- as.factor(as.Date(dimnames(data)[[3]],format="%Y_%m_%d"))

aperm(apply(data,c(1,2), by, group, mean),c(2,3,1))