Try this (assuming your time index is POSIXct):
library(zoo)
st <- as.POSIXct("2012-01-21 18:45")
g <- seq(st, end(z), by = "15 min") # grid
na.approx(z, xout = g)
See ?na.approx.zoo
for more info.
Note: Since the question did not provide the data in reproducible form we do so here:
Lines <- "Id date Time humid humtemp prtemp press t1
1 2012-01-21 18:41:50 47.7 14.12 13.870 1005.70 -0.05277778
1 2012-01-21 18:46:43 44.5 15.37 15.100 1005.20 0.02861111
1 2012-01-21 18:51:35 43.2 15.88 15.576 1005.10 0.10972222
1 2012-01-21 18:56:28 42.5 16.17 15.833 1004.90 0.19111111
1 2012-01-21 19:01:21 42.2 16.31 15.986 1004.80 0.27250000
1 2012-01-21 19:06:14 41.8 16.47 16.118 1004.60 0.35388889
1 2012-01-21 19:11:07 41.6 16.51 16.177 1004.60 0.43527778"
library(zoo)
z <- read.zoo(text = Lines, header = TRUE, index = 2:3, tz = "")
st <- as.POSIXct("2012-01-21 18:45")
g <- seq(st, end(z), by = "15 min") # grid
na.approx(z, xout = g)
giving:
Id humid humtemp prtemp press t1
2012-01-21 18:45:00 1 45.62491 14.93058 14.66761 1005.376 -1.501706e-09
2012-01-21 19:00:00 1 42.28294 16.27130 15.94370 1004.828 2.500000e-01