Appellant, the Bunge Corporation (“Bunge”), appeals from the judgment in favor of the Alabama State Docks Department in the amount of $30,298.16 plus interest. This amount represents the storage fee for Bunge’s corn, pursuant to a tariff issued by the Docks Department under the authority of the Shipping Act, 46 U.S.C.A. §§ 801-842 (1975 and Supp. 1981). Bunge protests the amount of the fee on the ground that the rate was changed during the period of storage. Bunge argues that it was improper to chárge Bunge a higher rate, as of the effective date of the new tariff, solely because Bunge’s corn had already been in storage for a length of time. Accordingly, Bunge maintains that it owes only $10,158.65 in storage fees. We affirm.
I. FACTS
The material facts are not in dispute. In the fall of 1977, Bunge stored a quantity of com in the grain elevators owned and operated by the Alabama State Docks Department in Mobile, Alabama. At that time, the Docks Department tariff provided that, following a ten-day free time, storage charges on grain would be calculated at the rate of one-tenth of a cent per bushel per day. Because the United States Department of Agriculture and the Food and Drug Administration suspected that the grain was contaminated with aflatoxin mold, the grain remained stored for an extended period of time. On May 6,1978, a new storage tariff became effective. The new tariff, called Item No. 5-A of Supplement No. 4, reads:
*66Storage Charges
Except as otherwise provided, storage charges for grain . . . , after the expiration of 10 days free time . . . will be assessed as follows:
Per Bu.
Per Day
1st 60-calendar-day period......... 1/10$
2nd 60-calendar-day period ........ 1/5$
3rd and each successive
60-calendar-day periods...........2/5$
The grain was still stored as of the effective date of this amendment and remained stored until July 24, 1978, when the last of it was removed. The Docks Department submitted a bill in the net amount of $30,-298.16, calculated at the rate of two-fifths cents per bushel per day for storage from May 6 to July 24, 1978. The Docks Department’s position was that since the grain had been stored for more than 120 consecutive days prior to May 6, 1978, the third or highest rate under the new tariff applied. Bunge maintains that this is an erroneous interpretation of the amended tariff. It believes that the first 60-day period at the Vio$ rate should commence to run from the effective date of the amendment and that, by its calculations, it owes only $10,158.65 in storage charges.
On cross motions for summary judgment, the district court held that “the rate periods are measured from the expiration of the free time, and not from any other date.” Accordingly, it adopted the Docks Department’s interpretation of the tariff and granted its motion for summary judgment. On appeal, Bunge challenges the district court’s interpretation of the tariff. In addition, it claims that the district court’s interpretation of the tariff violates the Shipping Act’s requirement that tariffs be “just and fair” in that Bunge was treated unequally when it was deprived of the lower rates for the first 120 days after May 6, 1978. Bunge also claims that the district court has given retroactive effect to its amended tariff, thereby violating its right to due process.
II. DISCUSSION
Our first task is to separate the issue of the proper interpretation of the tariff from the other issues of just and fair treatment and retroactivity. In general, the ordinary rules of contract interpretation apply in interpreting the language of tariffs. See Atchison, Topeka & Santa Fe Railway Co. v. United States, 572 F.2d 843, 849 (Ct.C1.1978). In certain cases, however, the rules of statutory construction also apply in interpreting tariffs. See State of Israel v. Metropolitan Dade County, Florida, 431 F.2d 925, 928-29 (5th Cir.1970). Looking solely to the amended tariff, we find ourselves in agreement with the district court that the tariff is not ambiguous and that the rate schedule contained therein applies after the expiration of ten days free time. Bunge argues that it should receive the benefit of the lower Vio$ rate for the first 60 days after May 6,1978, and that it should receive the benefit of the Vs$ rate for the next 60 days. Somewhat inconsistently, Bunge does not argue that it is entitled to a new 10 days free time as of the May 6, 1978, effective date of the new tariff. Bunge’s interpretation is clearly at odds with the language of the amendment itself. The amended tariff provides that “storage charges for grain . . . after expiration of 10 days free time .. . will be assessed as follows.” The tariff establishes the “expiration of 10 days free time” as the time from which the first 60 days for the lowest rate will run, etc. It is undisputed that Bunge’s free time expired sometime in September of 1977. Thus, the 120 days of potentially lower rates under the tariff had expired long before the May 6, 1978, effective date of the tariff. We conclude that the tariff unambiguously imposes the %$ rate upon Bunge from and after the May 6, 1978, effective date. This interpretation comports with the purpose of the tariff. The progressive rates were intended to provide an incentive to encourage storage for shorter periods, or to compensate the Docks Department for loss of revenue associated with slow-moving grain (e. g., loss of fees related to moving grain in and out of storage).
Next, Bunge argues that the new tariff violates the Shipping Act’s require*67ment that a tariff be “just and reasonable,” 46 U.S.C.A. §§ 816 and 817, by treating it differently from those who stored their grain on or after May 6, 1978. This argument is without merit. Any difference in treatment is due solely to the length of time for which the grain is stored. It is not unjust or unreasonable to impose a progressive rate schedule based upon length of storage time.
Finally, Bunge argues that the district court’s interpretation of the amended tariff results in a retroactive application of the higher rates in violation of the due process clause. We do not agree. In the realm of statutory construction, it is well established that a “statute is not retroactive merely because it draws upon antecedent facts for its operation.” Lewis v. Fidelity & Deposit Company of Maryland, 292 U.S. 559, 571, 54 S.Ct. 848, 853, 78 L.Ed. 1425 (1934); Lohf v. Casey, 466 F.2d 618 (10th Cir.1972); C.F. Industries v. Transcontinental Gas Pipeline, 448 F.Supp. 475 (W.D.N.C.1978). The antecedent fact upon which the tariff at issue draws is the fact that Bunge had already had its grain in storage for a lengthy time. It is neither unreasonable nor discriminatory to make a distinction between one who has already stored his goods for a lengthy time and one who has stored goods for a lesser time or for no time at all. The proper purpose of the progressive rate was to provide an incentive for shippers to store their grain for a shorter time, thus increasing the availability of the facility for other users desiring to export grain, and also to compensate for loss of revenues associated with slow-moving grain. We conclude that the tariff at issue is not retroactive merely because it makes a distinction in rates based upon the antecedent fact of the length of storage.1 See Selden & Co. v. Board of Trustees of the Galveston Wharves, 7 F.M.C. 679, 1964 A.M.C. 1621 (1964).
AFFIRMED.
. We note that the increased rate was charged commencing May 6, 1978, the effective date of the tariff. We are not dealing with an attempt to charge an increased rate commencing before the effective date. We note also that Bunge does not complain of any insufficiency relating to any lack of advance notice of the new tariff.