  Telecommunication data is being used increasingly in urban mobility
applications around the world. Despite its ubiquity and usefulness, technical
difficulties arise when using Packet-Switched Charging Data Records (CDR),
since its main purpose was not intended for this kind of applications. Due to
its particular nature, a trade-off must be considered between accessibility and
time accuracy when using this data. On the one hand, to obtain highly accurate
timestamps, huge amounts of network-level CDR must be extracted and stored.
This task is very difficult and expensive since highly critical network node
applications can be compromised in the data extraction and storage. On the
other hand, post-mediation CDR can be easily accessed since no network node
application is involved in its analysis. The pay-off is in the lower accurate
timestamps recorded, since several aggregations and filtering is performed in
previous steps of the charging pipelines. In this work, a detailed description
of the timestamp error problem using post-mediation CDR is presented, together
with a methodology to analyze error time series collected in each network cell.
