Code table 5- Time range indicator
from the ECMWF publication "Binary data representation - FM 92 GRIB", May 1992

Code 
figure  Meaning
0       Forecast product valid for reference time + P1 (P1 > 0), or
        Uninitialized analysis product for reference time (P1 = 0), or
        Image product for reference time (P1 = O)
1       Initialized analysis product for reference time (P1 = O)
2       Product with a valid time ranging between reference time + P1 and 
        reference time + P2
3       Average (reference time + P1 to reference time + P2)
4       Accumulation (reference time + P1 to reference time + P2) product
        considered valid at reference time + P2

5       Difference (reference time + P2 minus reference time + Pl) product 
        considered valid at reference time + P2
6       \
...         Reserved
9       /
10        P1 occupies octets 19 and 20; product valid at reference time + P1

11      \
...         Reserved
112     /

113     Average of N forecasts (or initialized analyses); each product has
        forecast period of P1 (P1 = 0 for initialized analyses); products have
        reference times at intervals of P2, beginning at the given reference 
        time
114     Accumulation of N forecasts (or initialized analyses); each product has
        forecast period of P1 (P1 = 0 for initialized analyses); products have
        reference times at intervals of P2, beginning at the given reference 
        time
115     Average of N forecasts, all with the same reference time; the first has
        a forecast period of Pl, the remaining forecasts follow at intervals of 
        P2
116     Accumulation of N forecasts, all with the same reference time; the 
        first has a forecast period of Pl, the remaining forecasts follow at 
        intervals of P2
117     Average of N forecasts; the first has a forecast period of Pl, the
        subsequent ones have forecast periods reduced from the previous one
        by an interval of P2; the reference time for the first is given in 
        octets 13
        to 17, the subsequent ones have reference times increased from the
        previous one by an interval of P2. Thus all the forecasts have the 
        same valid time, given by the initial reference time + P1
118     \
...      Reserved
122     /
123     Average of N uninitialized analyses, starting at the reference time, at
        intervals of P2
124     Accumulation of N uninitialized analyses, starting at the reference 
        time, at Intervals of P2
125     \
.
.        Reserved
.
254     /
Notes:

(1) For analysis products, or the first of a series of analysis products, 
the reference time (octets 13 to 17)indicates the valid time.

(2) For forecast products, or the first of a series of forecast products, 
the reference time indicates the valid time of the analysis upon which the 
(first) forecast is based.

(3) Initialized analysis products are allocated code numbers distinct from 
those allocated to uninitialized analysis products.

(4) Code figure 10 allows the period of a forecast to be extended over two 
octets; this is to assist with extended range forecasts.

(5) Where products or a series of products are averaged or accumulated, the 
number involved is to be represented in octets 22 and 23 of Section 1, while 
any number missing is to be represented in octet 24.

(6) Forecasts of the accumulation or difference of some quantity (e.g. 
quantitative precipitation forecasts), indicated by values of 4 or 5 in octet 
21, have a product valid time given by the reference time + P2; the period of 
accumulation, or difference, can be calculated as P2- Pl.

(7) A few examples may help to clarify the use of Code table 5:
For analysis products, P1 will be zero and the time range indicator will also 
be zero; for initialized products (sometimes called 'zero hour forecasts'), 
P1 will be zero, but octet 21 will be set to 1.
For forecasts, typically, P1 will contain the number of hours of the forecast 
(the unit indicator given in octet 18 would be 1) and octet 21 would contain a 
zero. Code value 115 would be used, typically, for multiple day mean forecasts, 
all derived from the same initial conditions.

Code value 117 would be used, typically, for Monte Carlo type calculations: 
many forecasts valid at the same time from different initial (reference) times.

Averages, accumulations and differences get a somewhat specialized treatment. 
If octet 21 (Code table 5) has a value between 2 and 5 (inclusive), then the 
reference time + P1 is the initial date/time and the reference time + P2 is 
the final date/time of the period over which averaging or accumulation takes 
place. If, however, octet 21 has a value of 113, 114, 115, 116, 117, 123 or 
124, then P2 specifies the time interval between each of the fields (or the 
forecast initial times) that have been averaged or accumulated. These latter 
values of octet 21 require the qualities averaged to be equally separated in 
time; the former values, 3 and 4 in particular, allow for irregular or 
unspecified intervals of time between the fields that are averaged or 
accumulated.