This may seem like a trivial task, but the retention times you measure on one HPLC instrument can be very different from those measured using a different HPLC instrument...even when using exactly the same column!
Fortunately the instrument-dependent causes of these retention time shifts are well-known. They result from the following differences between HPLC instruments:
a) Solvent mis-proportioning
HPLC systems are normally specified to deliver solvent proportioning to within 1% of the set value - this is enough to cause huge changes in retention. For example, if you consider a typical solute eluting at the end of a 20 min linear gradient going from 10% B to 100% B, a 1% solvent proportioning error could account for a difference of more than 0.48 min (2.4% of the gradient time).
b) Flow rate
The theoretical flow rate error from water/acetonitrile gradients is up to 3%, though experimental measurements have shown up to 8% error. There are many sources for this error including non-zero volume of mixing, non-zero compressibility, pressure-dependent viscosities, and others. During a gradient, the flow rate vs. time profile can be quite different between different instruments, particularly between pumps that employ high-pressure mixing vs. those that employ low-pressure mixing.
c) Gradient dispersion
Gradient dispersion is characterized by "rounding" of the gradient profile at points where the gradient slope changes (in a linear gradient, this would be at the beginning and end of the gradient). At low flow rates, rounding can be very pronounced and differs significantly between HPLC instruments.
d) Dwell time
Last, but certainly not least, is the dwell time. This is the time it takes for the programmed gradient to actually get to the column inlet. This delay depends on how much volume is between the pump mixer and the column inlet - something that is highly variable between HPLC instruments (though it can be controlled pretty easily by making modifications to the instrument).