Variable entry times for first occasion

[Michlers]

Hi

I am analyzing data on post-fledging survival over a 6 month period.
Observations were started directly after the first birds had fledged. Therefore, individuals were still entering the population during the first two months of our study. As shorter intervals gave problems with parameter estimation I now use monthly intervals where birds enter the population at the end of the month they fledged in. However, this creates a problem as those individuals that fledge earlier or later within a month are treated equally. An earlier advice suggested using an individual covariate (the number of days the individual is available during the first interval) which should only be used for phi estimates of the first diagonal of the PIM. However, I am not sure of how to set that up in the phi PIM and the DM. Do I also need to specify different age classes (birds in first month and later)?

Thanks!